# Date and Time

This chapter describes functions for manipulating dates and times, including functions for determining what time it is and conversion between different time representations.

## Time Basics

Discussing time in a technical manual can be difficult because the word "time" in English refers to lots of different things. In this manual, we use a rigorous terminology to avoid confusion, and the only thing we use the simple word "time" for is to talk about the abstract concept.

A calendar time is a point in the time continuum, for example November 4, 1990 at 18:02.5 UTC. Sometimes this is called "absolute time".

We don't speak of a "date", because that is inherent in a calendar time.

An interval is a contiguous part of the time continuum between two calendar times, for example the hour between 9:00 and 10:00 on July 4, 1980.

An elapsed time is the length of an interval, for example, 35 minutes. People sometimes sloppily use the word "interval" to refer to the elapsed time of some interval.

An amount of time is a sum of elapsed times, which need not be of any specific intervals. For example, the amount of time it takes to read a book might be 9 hours, independently of when and in how many sittings it is read.

A period is the elapsed time of an interval between two events, especially when they are part of a sequence of regularly repeating events.

CPU time is like calendar time, except that it is based on the subset of the time continuum when a particular process is actively using a CPU. CPU time is, therefore, relative to a process.

Processor time is an amount of time that a CPU is in use. In fact, it's a basic system resource, since there's a limit to how much can exist in any given interval (that limit is the elapsed time of the interval times the number of CPUs in the processor). People often call this CPU time, but we reserve the latter term in this manual for the definition above.

## Elapsed Time

One way to represent an elapsed time is with a simple arithmetic data type, as with the following function to compute the elapsed time between two calendar times. This function is declared in `time.h'.

Function: double difftime (time_t time1, time_t time0)
The `difftime` function returns the number of seconds of elapsed time between calendar time time1 and calendar time time0, as a value of type `double`. The difference ignores leap seconds unless leap second support is enabled.

In the GNU system, you can simply subtract `time_t` values. But on other systems, the `time_t` data type might use some other encoding where subtraction doesn't work directly.

The GNU C library provides two data types specifically for representing an elapsed time. They are used by various GNU C library functions, and you can use them for your own purposes too. They're exactly the same except that one has a resolution in microseconds, and the other, newer one, is in nanoseconds.

Data Type: struct timeval
The `struct timeval` structure represents an elapsed time. It is declared in `sys/time.h' and has the following members:

`long int tv_sec`
This represents the number of whole seconds of elapsed time.
`long int tv_usec`
This is the rest of the elapsed time (a fraction of a second), represented as the number of microseconds. It is always less than one million.

Data Type: struct timespec
The `struct timespec` structure represents an elapsed time. It is declared in `time.h' and has the following members:

`long int tv_sec`
This represents the number of whole seconds of elapsed time.
`long int tv_nsec`
This is the rest of the elapsed time (a fraction of a second), represented as the number of nanoseconds. It is always less than one billion.

It is often necessary to subtract two values of type ```struct timeval``` or `struct timespec`. Here is the best way to do this. It works even on some peculiar operating systems where the `tv_sec` member has an unsigned type.

```/* Subtract the `struct timeval' values X and Y,
storing the result in RESULT.
Return 1 if the difference is negative, otherwise 0.  */

int
timeval_subtract (result, x, y)
struct timeval *result, *x, *y;
{
/* Perform the carry for the later subtraction by updating y. */
if (x->tv_usec < y->tv_usec) {
int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
y->tv_usec -= 1000000 * nsec;
y->tv_sec += nsec;
}
if (x->tv_usec - y->tv_usec > 1000000) {
int nsec = (x->tv_usec - y->tv_usec) / 1000000;
y->tv_usec += 1000000 * nsec;
y->tv_sec -= nsec;
}

/* Compute the time remaining to wait.
`tv_usec` is certainly positive. */
result->tv_sec = x->tv_sec - y->tv_sec;
result->tv_usec = x->tv_usec - y->tv_usec;

/* Return 1 if result is negative. */
return x->tv_sec < y->tv_sec;
}
```

Common functions that use `struct timeval` are `gettimeofday` and `settimeofday`.

There are no GNU C library functions specifically oriented toward dealing with elapsed times, but the calendar time, processor time, and alarm and sleeping functions have a lot to do with them.

## Processor And CPU Time

If you're trying to optimize your program or measure its efficiency, it's very useful to know how much processor time it uses. For that, calendar time and elapsed times are useless because a process may spend time waiting for I/O or for other processes to use the CPU. However, you can get the information with the functions in this section.

CPU time (see section Time Basics) is represented by the data type `clock_t`, which is a number of clock ticks. It gives the total amount of time a process has actively used a CPU since some arbitrary event. On the GNU system, that event is the creation of the process. While arbitrary in general, the event is always the same event for any particular process, so you can always measure how much time on the CPU a particular computation takes by examinining the process' CPU time before and after the computation.

In the GNU system, `clock_t` is equivalent to `long int` and `CLOCKS_PER_SEC` is an integer value. But in other systems, both `clock_t` and the macro `CLOCKS_PER_SEC` can be either integer or floating-point types. Casting CPU time values to `double`, as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is.

Note that the clock can wrap around. On a 32bit system with `CLOCKS_PER_SEC` set to one million this function will return the same value approximately every 72 minutes.

For additional functions to examine a process' use of processor time, and to control it, See section Resource Usage And Limitation.

### CPU Time Inquiry

To get a process' CPU time, you can use the `clock` function. This facility is declared in the header file `time.h'.

In typical usage, you call the `clock` function at the beginning and end of the interval you want to time, subtract the values, and then divide by `CLOCKS_PER_SEC` (the number of clock ticks per second) to get processor time, like this:

```#include <time.h>

clock_t start, end;
double cpu_time_used;

start = clock();
... /* Do the work. */
end = clock();
cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
```

Do not use a single CPU time as an amount of time; it doesn't work that way. Either do a subtraction as shown above or query processor time directly. See section Processor Time Inquiry.

Different computers and operating systems vary wildly in how they keep track of CPU time. It's common for the internal processor clock to have a resolution somewhere between a hundredth and millionth of a second.

Macro: int CLOCKS_PER_SEC
The value of this macro is the number of clock ticks per second measured by the `clock` function. POSIX requires that this value be one million independent of the actual resolution.

Macro: int CLK_TCK
This is an obsolete name for `CLOCKS_PER_SEC`.

Data Type: clock_t
This is the type of the value returned by the `clock` function. Values of type `clock_t` are numbers of clock ticks.

Function: clock_t clock (void)
This function returns the calling process' current CPU time. If the CPU time is not available or cannot be represented, `clock` returns the value `(clock_t)(-1)`.

### Processor Time Inquiry

The `times` function returns information about a process' consumption of processor time in a `struct tms` object, in addition to the process' CPU time. See section Time Basics. You should include the header file `sys/times.h' to use this facility.

Data Type: struct tms
The `tms` structure is used to return information about process times. It contains at least the following members:

`clock_t tms_utime`
This is the total processor time the calling process has used in executing the instructions of its program.
`clock_t tms_stime`
This is the processor time the system has used on behalf of the calling process.
`clock_t tms_cutime`
This is the sum of the `tms_utime` values and the `tms_cutime` values of all terminated child processes of the calling process, whose status has been reported to the parent process by `wait` or `waitpid`; see section Process Completion. In other words, it represents the total processor time used in executing the instructions of all the terminated child processes of the calling process, excluding child processes which have not yet been reported by `wait` or `waitpid`.
`clock_t tms_cstime`
This is similar to `tms_cutime`, but represents the total processor time system has used on behalf of all the terminated child processes of the calling process.

All of the times are given in numbers of clock ticks. Unlike CPU time, these are the actual amounts of time; not relative to any event. See section Creating a Process.

Function: clock_t times (struct tms *buffer)
The `times` function stores the processor time information for the calling process in buffer.

The return value is the calling process' CPU time (the same value you get from `clock()`. `times` returns `(clock_t)(-1)` to indicate failure.

Portability Note: The `clock` function described in section CPU Time Inquiry is specified by the ISO C standard. The `times` function is a feature of POSIX.1. In the GNU system, the CPU time is defined to be equivalent to the sum of the `tms_utime` and `tms_stime` fields returned by `times`.

## Calendar Time

This section describes facilities for keeping track of calendar time. See section Time Basics.

The GNU C library represents calendar time three ways:

• Simple time (the `time_t` data type) is a compact representation, typically giving the number of seconds of elapsed time since some implementation-specific base time.
• There is also a "high-resolution time" representation. Like simple time, this represents a calendar time as an elapsed time since a base time, but instead of measuring in whole seconds, it uses a ```struct timeval``` data type, which includes fractions of a second. Use this time representation instead of simple time when you need greater precision.
• Local time or broken-down time (the `struct tm` data type) represents a calendar time as a set of components specifying the year, month, and so on in the Gregorian calendar, for a specific time zone. This calendar time representation is usually used only to communicate with people.

### Simple Calendar Time

This section describes the `time_t` data type for representing calendar time as simple time, and the functions which operate on simple time objects. These facilities are declared in the header file `time.h'.

Data Type: time_t
This is the data type used to represent simple time. Sometimes, it also represents an elapsed time. When interpreted as a calendar time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time. (This calendar time is sometimes referred to as the epoch.) POSIX requires that this count not include leap seconds, but on some systems this count includes leap seconds if you set `TZ` to certain values (see section Specifying the Time Zone with `TZ`).

Note that a simple time has no concept of local time zone. Calendar Time T is the same instant in time regardless of where on the globe the computer is.

In the GNU C library, `time_t` is equivalent to `long int`. In other systems, `time_t` might be either an integer or floating-point type.

The function `difftime` tells you the elapsed time between two simple calendar times, which is not always as easy to compute as just subtracting. See section Elapsed Time.

Function: time_t time (time_t *result)
The `time` function returns the current calendar time as a value of type `time_t`. If the argument result is not a null pointer, the calendar time value is also stored in `*result`. If the current calendar time is not available, the value `(time_t)(-1)` is returned.

Function: int stime (time_t *newtime)
`stime` sets the system clock, i.e. it tells the system that the current calendar time is newtime, where `newtime` is interpreted as described in the above definition of `time_t`.

`settimeofday` is a newer function which sets the system clock to better than one second precision. `settimeofday` is generally a better choice than `stime`. See section High-Resolution Calendar.

Only the superuser can set the system clock.

If the function succeeds, the return value is zero. Otherwise, it is `-1` and `errno` is set accordingly:

`EPERM`
The process is not superuser.

### High-Resolution Calendar

The `time_t` data type used to represent simple times has a resolution of only one second. Some applications need more precision.

So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in `sys/time.h'.

Data Type: struct timezone
The `struct timezone` structure is used to hold minimal information about the local time zone. It has the following members:

`int tz_minuteswest`
This is the number of minutes west of UTC.
`int tz_dsttime`
If nonzero, Daylight Saving Time applies during some part of the year.

The `struct timezone` type is obsolete and should never be used. Instead, use the facilities described in section Functions and Variables for Time Zones.

Function: int gettimeofday (struct timeval *tp, struct timezone *tzp)
The `gettimeofday` function returns the current calendar time as the elapsed time since the epoch in the `struct timeval` structure indicated by tp. (see section Elapsed Time for a description of `struct timespec`). Information about the time zone is returned in the structure pointed at tzp. If the tzp argument is a null pointer, time zone information is ignored.

The return value is `0` on success and `-1` on failure. The following `errno` error condition is defined for this function:

`ENOSYS`
The operating system does not support getting time zone information, and tzp is not a null pointer. The GNU operating system does not support using `struct timezone` to represent time zone information; that is an obsolete feature of 4.3 BSD. Instead, use the facilities described in section Functions and Variables for Time Zones.

Function: int settimeofday (const struct timeval *tp, const struct timezone *tzp)
The `settimeofday` function sets the current calendar time in the system clock according to the arguments. As for `gettimeofday`, the calendar time is represented as the elapsed time since the epoch. As for `gettimeofday`, time zone information is ignored if tzp is a null pointer.

You must be a privileged user in order to use `settimeofday`.

Some kernels automatically set the system clock from some source such as a hardware clock when they start up. Others, including Linux, place the system clock in an "invalid" state (in which attempts to read the clock fail). A call of `stime` removes the system clock from an invalid state, and system startup scripts typically run a program that calls `stime`.

`settimeofday` causes a sudden jump forwards or backwards, which can cause a variety of problems in a system. Use `adjtime` (below) to make a smooth transition from one time to another by temporarily speeding up or slowing down the clock.

With a Linux kernel, `adjtimex` does the same thing and can also make permanent changes to the speed of the system clock so it doesn't need to be corrected as often.

The return value is `0` on success and `-1` on failure. The following `errno` error conditions are defined for this function:

`EPERM`
This process cannot set the clock because it is not privileged.
`ENOSYS`
The operating system does not support setting time zone information, and tzp is not a null pointer.

Function: int adjtime (const struct timeval *delta, struct timeval *olddelta)
This function speeds up or slows down the system clock in order to make a gradual adjustment. This ensures that the calendar time reported by the system clock is always monotonically increasing, which might not happen if you simply set the clock.

The delta argument specifies a relative adjustment to be made to the clock time. If negative, the system clock is slowed down for a while until it has lost this much elapsed time. If positive, the system clock is speeded up for a while.

If the olddelta argument is not a null pointer, the `adjtime` function returns information about any previous time adjustment that has not yet completed.

This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it.

With a Linux kernel, you can use the `adjtimex` function to permanently change the clock speed.

The return value is `0` on success and `-1` on failure. The following `errno` error condition is defined for this function:

`EPERM`
You do not have privilege to set the time.

Portability Note: The `gettimeofday`, `settimeofday`, and `adjtime` functions are derived from BSD.

Symbols for the following function are declared in `sys/timex.h'.

Function: int adjtimex (struct timex *timex)

`adjtimex` is functionally identical to `ntp_adjtime`. See section High Accuracy Clock.

This function is present only with a Linux kernel.

### Broken-down Time

Calendar time is represented by the usual GNU C library functions as an elapsed time since a fixed base calendar time. This is convenient for computation, but has no relation to the way people normally think of calendar time. By contrast, broken-down time is a binary representation of calendar time separated into year, month, day, and so on. Broken-down time values are not useful for calculations, but they are useful for printing human readable time information.

A broken-down time value is always relative to a choice of time zone, and it also indicates which time zone that is.

The symbols in this section are declared in the header file `time.h'.

Data Type: struct tm
This is the data type used to represent a broken-down time. The structure contains at least the following members, which can appear in any order.

`int tm_sec`
This is the number of full seconds since the top of the minute (normally in the range `0` through `59`, but the actual upper limit is `60`, to allow for leap seconds if leap second support is available).
`int tm_min`
This is the number of full minutes since the top of the hour (in the range `0` through `59`).
`int tm_hour`
This is the number of full hours past midnight (in the range `0` through `23`).
`int tm_mday`
This is the ordinal day of the month (in the range `1` through `31`). Watch out for this one! As the only ordinal number in the structure, it is inconsistent with the rest of the structure.
`int tm_mon`
This is the number of full calendar months since the beginning of the year (in the range `0` through `11`). Watch out for this one! People usually use ordinal numbers for month-of-year (where January = 1).
`int tm_year`
This is the number of full calendar years since 1900.
`int tm_wday`
This is the number of full days since Sunday (in the range `0` through `6`).
`int tm_yday`
This is the number of full days since the beginning of the year (in the range `0` through `365`).
`int tm_isdst`
This is a flag that indicates whether Daylight Saving Time is (or was, or will be) in effect at the time described. The value is positive if Daylight Saving Time is in effect, zero if it is not, and negative if the information is not available.
`long int tm_gmtoff`
This field describes the time zone that was used to compute this broken-down time value, including any adjustment for daylight saving; it is the number of seconds that you must add to UTC to get local time. You can also think of this as the number of seconds east of UTC. For example, for U.S. Eastern Standard Time, the value is `-5*60*60`. The `tm_gmtoff` field is derived from BSD and is a GNU library extension; it is not visible in a strict ISO C environment.
`const char *tm_zone`
This field is the name for the time zone that was used to compute this broken-down time value. Like `tm_gmtoff`, this field is a BSD and GNU extension, and is not visible in a strict ISO C environment.

Function: struct tm * localtime (const time_t *time)
The `localtime` function converts the simple time pointed to by time to broken-down time representation, expressed relative to the user's specified time zone.

The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to `ctime`, `gmtime`, or `localtime`. (But no other library function overwrites the contents of this object.)

The return value is the null pointer if time cannot be represented as a broken-down time; typically this is because the year cannot fit into an `int`.

Calling `localtime` has one other effect: it sets the variable `tzname` with information about the current time zone. See section Functions and Variables for Time Zones.

Using the `localtime` function is a big problem in multi-threaded programs. The result is returned in a static buffer and this is used in all threads. POSIX.1c introduced a variant of this function.

Function: struct tm * localtime_r (const time_t *time, struct tm *resultp)
The `localtime_r` function works just like the `localtime` function. It takes a pointer to a variable containing a simple time and converts it to the broken-down time format.

But the result is not placed in a static buffer. Instead it is placed in the object of type `struct tm` to which the parameter resultp points.

If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

Function: struct tm * gmtime (const time_t *time)
This function is similar to `localtime`, except that the broken-down time is expressed as Coordinated Universal Time (UTC) (formerly called Greenwich Mean Time (GMT)) rather than relative to a local time zone.

As for the `localtime` function we have the problem that the result is placed in a static variable. POSIX.1c also provides a replacement for `gmtime`.

Function: struct tm * gmtime_r (const time_t *time, struct tm *resultp)
This function is similar to `localtime_r`, except that it converts just like `gmtime` the given time as Coordinated Universal Time.

If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

Function: time_t mktime (struct tm *brokentime)
The `mktime` function is used to convert a broken-down time structure to a simple time representation. It also "normalizes" the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components.

The `mktime` function ignores the specified contents of the `tm_wday` and `tm_yday` members of the broken-down time structure. It uses the values of the other components to determine the calendar time; it's permissible for these components to have unnormalized values outside their normal ranges. The last thing that `mktime` does is adjust the components of the brokentime structure (including the `tm_wday` and `tm_yday`).

If the specified broken-down time cannot be represented as a simple time, `mktime` returns a value of `(time_t)(-1)` and does not modify the contents of brokentime.

Calling `mktime` also sets the variable `tzname` with information about the current time zone. See section Functions and Variables for Time Zones.

Function: time_t timelocal (struct tm *brokentime)

`timelocal` is functionally identical to `mktime`, but more mnemonically named. Note that it is the inverse of the `localtime` function.

Portability note: `mktime` is essentially universally available. `timelocal` is rather rare.

Function: time_t timegm (struct tm *brokentime)

`timegm` is functionally identical to `mktime` except it always takes the input values to be Coordinated Universal Time (UTC) regardless of any local time zone setting.

Note that `timegm` is the inverse of `gmtime`.

Portability note: `mktime` is essentially universally available. `timegm` is rather rare. For the most portable conversion from a UTC broken-down time to a simple time, set the `TZ` environment variable to UTC, call `mktime`, then set `TZ` back.

### High Accuracy Clock

The `ntp_gettime` and `ntp_adjtime` functions provide an interface to monitor and manipulate the system clock to maintain high accuracy time. For example, you can fine tune the speed of the clock or synchronize it with another time source.

A typical use of these functions is by a server implementing the Network Time Protocol to synchronize the clocks of multiple systems and high precision clocks.

These functions are declared in `sys/timex.h'.

Data Type: struct ntptimeval
This structure is used for information about the system clock. It contains the following members:
`struct timeval time`
This is the current calendar time, expressed as the elapsed time since the epoch. The `struct timeval` data type is described in section Elapsed Time.
`long int maxerror`
This is the maximum error, measured in microseconds. Unless updated via `ntp_adjtime` periodically, this value will reach some platform-specific maximum value.
`long int esterror`
This is the estimated error, measured in microseconds. This value can be set by `ntp_adjtime` to indicate the estimated offset of the system clock from the true calendar time.

Function: int ntp_gettime (struct ntptimeval *tptr)
The `ntp_gettime` function sets the structure pointed to by tptr to current values. The elements of the structure afterwards contain the values the timer implementation in the kernel assumes. They might or might not be correct. If they are not a `ntp_adjtime` call is necessary.

The return value is `0` on success and other values on failure. The following `errno` error conditions are defined for this function:

`TIME_ERROR`
The precision clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care.

Data Type: struct timex
This structure is used to control and monitor the system clock. It contains the following members:
`unsigned int modes`
This variable controls whether and which values are set. Several symbolic constants have to be combined with binary or to specify the effective mode. These constants start with `MOD_`.
`long int offset`
This value indicates the current offset of the system clock from the true calendar time. The value is given in microseconds. If bit `MOD_OFFSET` is set in `modes`, the offset (and possibly other dependent values) can be set. The offset's absolute value must not exceed `MAXPHASE`.
`long int frequency`
This value indicates the difference in frequency between the true calendar time and the system clock. The value is expressed as scaled PPM (parts per million, 0.0001%). The scaling is ```1 << SHIFT_USEC```. The value can be set with bit `MOD_FREQUENCY`, but the absolute value must not exceed `MAXFREQ`.
`long int maxerror`
This is the maximum error, measured in microseconds. A new value can be set using bit `MOD_MAXERROR`. Unless updated via `ntp_adjtime` periodically, this value will increase steadily and reach some platform-specific maximum value.
`long int esterror`
This is the estimated error, measured in microseconds. This value can be set using bit `MOD_ESTERROR`.
`int status`
This variable reflects the various states of the clock machinery. There are symbolic constants for the significant bits, starting with `STA_`. Some of these flags can be updated using the `MOD_STATUS` bit.
`long int constant`
This value represents the bandwidth or stiffness of the PLL (phase locked loop) implemented in the kernel. The value can be changed using bit `MOD_TIMECONST`.
`long int precision`
This value represents the accuracy or the maximum error when reading the system clock. The value is expressed in microseconds.
`long int tolerance`
This value represents the maximum frequency error of the system clock in scaled PPM. This value is used to increase the `maxerror` every second.
`struct timeval time`
The current calendar time.
`long int tick`
The elapsed time between clock ticks in microseconds. A clock tick is a periodic timer interrupt on which the system clock is based.
`long int ppsfreq`
This is the first of a few optional variables that are present only if the system clock can use a PPS (pulse per second) signal to discipline the system clock. The value is expressed in scaled PPM and it denotes the difference in frequency between the system clock and the PPS signal.
`long int jitter`
This value expresses a median filtered average of the PPS signal's dispersion in microseconds.
`int shift`
This value is a binary exponent for the duration of the PPS calibration interval, ranging from `PPS_SHIFT` to `PPS_SHIFTMAX`.
`long int stabil`
This value represents the median filtered dispersion of the PPS frequency in scaled PPM.
`long int jitcnt`
This counter represents the number of pulses where the jitter exceeded the allowed maximum `MAXTIME`.
`long int calcnt`
This counter reflects the number of successful calibration intervals.
`long int errcnt`
This counter represents the number of calibration errors (caused by large offsets or jitter).
`long int stbcnt`
This counter denotes the number of of calibrations where the stability exceeded the threshold.

Function: int ntp_adjtime (struct timex *tptr)
The `ntp_adjtime` function sets the structure specified by tptr to current values.

In addition, `ntp_adjtime` updates some settings to match what you pass to it in *tptr. Use the `modes` element of *tptr to select what settings to update. You can set `offset`, `freq`, `maxerror`, `esterror`, `status`, `constant`, and `tick`.

`modes` = zero means set nothing.

Only the superuser can update settings.

The return value is `0` on success and other values on failure. The following `errno` error conditions are defined for this function:

`TIME_ERROR`
The high accuracy clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care. Another reason could be that the specified new values are not allowed.
`EPERM`
The process specified a settings update, but is not superuser.

For more details see RFC1305 (Network Time Protocol, Version 3) and related documents.

Portability note: Early versions of the GNU C library did not have this function but did have the synonymous `adjtimex`.

### Formatting Calendar Time

The functions described in this section format calendar time values as strings. These functions are declared in the header file `time.h'.

Function: char * asctime (const struct tm *brokentime)
The `asctime` function converts the broken-down time value that brokentime points to into a string in a standard format:

```"Tue May 21 13:46:22 1991\n"
```

The abbreviations for the days of week are: `Sun', `Mon', `Tue', `Wed', `Thu', `Fri', and `Sat'.

The abbreviations for the months are: `Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'.

The return value points to a statically allocated string, which might be overwritten by subsequent calls to `asctime` or `ctime`. (But no other library function overwrites the contents of this string.)

Function: char * asctime_r (const struct tm *brokentime, char *buffer)
This function is similar to `asctime` but instead of placing the result in a static buffer it writes the string in the buffer pointed to by the parameter buffer. This buffer should have room for at least 26 bytes, including the terminating null.

If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns buffer. Otherwise return `NULL`.

Function: char * ctime (const time_t *time)
The `ctime` function is similar to `asctime`, except that you specify the calendar time argument as a `time_t` simple time value rather than in broken-down local time format. It is equivalent to

```asctime (localtime (time))
```

`ctime` sets the variable `tzname`, because `localtime` does so. See section Functions and Variables for Time Zones.

Function: char * ctime_r (const time_t *time, char *buffer)
This function is similar to `ctime`, but places the result in the string pointed to by buffer. It is equivalent to (written using gcc extensions, see section `Statement Exprs' in Porting and Using gcc):

```({ struct tm tm; asctime_r (localtime_r (time, &tm), buf); })
```

If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns buffer. Otherwise return `NULL`.

Function: size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)
This function is similar to the `sprintf` function (see section Formatted Input), but the conversion specifications that can appear in the format template template are specialized for printing components of the date and time brokentime according to the locale currently specified for time conversion (see section Locales and Internationalization).

Ordinary characters appearing in the template are copied to the output string s; this can include multibyte character sequences. Conversion specifiers are introduced by a `%' character, followed by an optional flag which can be one of the following. These flags are all GNU extensions. The first three affect only the output of numbers:

`_`
The number is padded with spaces.
`-`
The number is not padded at all.
`0`
The number is padded with zeros even if the format specifies padding with spaces.
`^`
The output uses uppercase characters, but only if this is possible (see section Case Conversion).

The default action is to pad the number with zeros to keep it a constant width. Numbers that do not have a range indicated below are never padded, since there is no natural width for them.

Following the flag an optional specification of the width is possible. This is specified in decimal notation. If the natural size of the output is of the field has less than the specified number of characters, the result is written right adjusted and space padded to the given size.

An optional modifier can follow the optional flag and width specification. The modifiers, which are POSIX.2 extensions, are:

`E`
Use the locale's alternate representation for date and time. This modifier applies to the `%c`, `%C`, `%x`, `%X`, `%y` and `%Y` format specifiers. In a Japanese locale, for example, `%Ex` might yield a date format based on the Japanese Emperors' reigns.
`O`
Use the locale's alternate numeric symbols for numbers. This modifier applies only to numeric format specifiers.

If the format supports the modifier but no alternate representation is available, it is ignored.

The conversion specifier ends with a format specifier taken from the following list. The whole `%' sequence is replaced in the output string as follows:

`%a`
The abbreviated weekday name according to the current locale.
`%A`
The full weekday name according to the current locale.
`%b`
The abbreviated month name according to the current locale.
`%B`
The full month name according to the current locale.
`%c`
The preferred calendar time representation for the current locale.
`%C`
The century of the year. This is equivalent to the greatest integer not greater than the year divided by 100. This format is a POSIX.2 extension and also appears in ISO C99.
`%d`
The day of the month as a decimal number (range `01` through `31`).
`%D`
The date using the format `%m/%d/%y`. This format is a POSIX.2 extension and also appears in ISO C99.
`%e`
The day of the month like with `%d`, but padded with blank (range ` 1` through `31`). This format is a POSIX.2 extension and also appears in ISO C99.
`%F`
The date using the format `%Y-%m-%d`. This is the form specified in the ISO 8601 standard and is the preferred form for all uses. This format is a ISO C99 extension.
`%g`
The year corresponding to the ISO week number, but without the century (range `00` through `99`). This has the same format and value as `%y`, except that if the ISO week number (see `%V`) belongs to the previous or next year, that year is used instead. This format was introduced in ISO C99.
`%G`
The year corresponding to the ISO week number. This has the same format and value as `%Y`, except that if the ISO week number (see `%V`) belongs to the previous or next year, that year is used instead. This format was introduced in ISO C99 but was previously available as a GNU extension.
`%h`
The abbreviated month name according to the current locale. The action is the same as for `%b`. This format is a POSIX.2 extension and also appears in ISO C99.
`%H`
The hour as a decimal number, using a 24-hour clock (range `00` through `23`).
`%I`
The hour as a decimal number, using a 12-hour clock (range `01` through `12`).
`%j`
The day of the year as a decimal number (range `001` through `366`).
`%k`
The hour as a decimal number, using a 24-hour clock like `%H`, but padded with blank (range ` 0` through `23`). This format is a GNU extension.
`%l`
The hour as a decimal number, using a 12-hour clock like `%I`, but padded with blank (range ` 1` through `12`). This format is a GNU extension.
`%m`
The month as a decimal number (range `01` through `12`).
`%M`
The minute as a decimal number (range `00` through `59`).
`%n`
A single `\n' (newline) character. This format is a POSIX.2 extension and also appears in ISO C99.
`%p`
Either `AM' or `PM', according to the given time value; or the corresponding strings for the current locale. Noon is treated as `PM' and midnight as `AM'.
`%P`
Either `am' or `pm', according to the given time value; or the corresponding strings for the current locale, printed in lowercase characters. Noon is treated as `pm' and midnight as `am'. This format was introduced in ISO C99 but was previously available as a GNU extension.
`%r`
The complete calendar time using the AM/PM format of the current locale. This format is a POSIX.2 extension and also appears in ISO C99.
`%R`
The hour and minute in decimal numbers using the format `%H:%M`. This format was introduced in ISO C99 but was previously available as a GNU extension.
`%s`
The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available. This format is a GNU extension.
`%S`
The seconds as a decimal number (range `00` through `60`).
`%t`
A single `\t' (tabulator) character. This format is a POSIX.2 extension and also appears in ISO C99.
`%T`
The time of day using decimal numbers using the format `%H:%M:%S`. This format is a POSIX.2 extension.
`%u`
The day of the week as a decimal number (range `1` through `7`), Monday being `1`. This format is a POSIX.2 extension and also appears in ISO C99.
`%U`
The week number of the current year as a decimal number (range `00` through `53`), starting with the first Sunday as the first day of the first week. Days preceding the first Sunday in the year are considered to be in week `00`.
`%V`
The ISO 8601:1988 week number as a decimal number (range `01` through `53`). ISO weeks start with Monday and end with Sunday. Week `01` of a year is the first week which has the majority of its days in that year; this is equivalent to the week containing the year's first Thursday, and it is also equivalent to the week containing January 4. Week `01` of a year can contain days from the previous year. The week before week `01` of a year is the last week (`52` or `53`) of the previous year even if it contains days from the new year. This format is a POSIX.2 extension and also appears in ISO C99.
`%w`
The day of the week as a decimal number (range `0` through `6`), Sunday being `0`.
`%W`
The week number of the current year as a decimal number (range `00` through `53`), starting with the first Monday as the first day of the first week. All days preceding the first Monday in the year are considered to be in week `00`.
`%x`
The preferred date representation for the current locale.
`%X`
The preferred time of day representation for the current locale.
`%y`
The year without a century as a decimal number (range `00` through `99`). This is equivalent to the year modulo 100.
`%Y`
The year as a decimal number, using the Gregorian calendar. Years before the year `1` are numbered `0`, `-1`, and so on.
`%z`
RFC 822/ISO 8601:1988 style numeric time zone (e.g., `-0600` or `+0100`), or nothing if no time zone is determinable. This format was introduced in ISO C99 but was previously available as a GNU extension. A full RFC 822 timestamp is generated by the format `"%a, %d %b %Y %H:%M:%S %z"' (or the equivalent `"%a, %d %b %Y %T %z"').
`%Z`
The time zone abbreviation (empty if the time zone can't be determined).
`%%`
A literal `%' character.

The size parameter can be used to specify the maximum number of characters to be stored in the array s, including the terminating null character. If the formatted time requires more than size characters, `strftime` returns zero and the contents of the array s are undefined. Otherwise the return value indicates the number of characters placed in the array s, not including the terminating null character.

Warning: This convention for the return value which is prescribed in ISO C can lead to problems in some situations. For certain format strings and certain locales the output really can be the empty string and this cannot be discovered by testing the return value only. E.g., in most locales the AM/PM time format is not supported (most of the world uses the 24 hour time representation). In such locales `"%p"` will return the empty string, i.e., the return value is zero. To detect situations like this something similar to the following code should be used:

```buf[0] = '\1';
len = strftime (buf, bufsize, format, tp);
if (len == 0 && buf[0] != '\0')
{
/* Something went wrong in the strftime call.  */
...
}
```

If s is a null pointer, `strftime` does not actually write anything, but instead returns the number of characters it would have written.

According to POSIX.1 every call to `strftime` implies a call to `tzset`. So the contents of the environment variable `TZ` is examined before any output is produced.

For an example of `strftime`, see section Time Functions Example.

Function: size_t wcsftime (wchar_t *s, size_t size, const wchar_t *template, const struct tm *brokentime)
The `wcsftime` function is equivalent to the `strftime` function with the difference that it operates on wide character strings. The buffer where the result is stored, pointed to by s, must be an array of wide characters. The parameter size which specifies the size of the output buffer gives the number of wide character, not the number of bytes.

Also the format string template is a wide character string. Since all characters needed to specify the format string are in the basic character set it is portably possible to write format strings in the C source code using the `L"..."` notation. The parameter brokentime has the same meaning as in the `strftime` call.

The `wcsftime` function supports the same flags, modifiers, and format specifiers as the `strftime` function.

The return value of `wcsftime` is the number of wide characters stored in `s`. When more characters would have to be written than can be placed in the buffer s the return value is zero, with the same problems indicated in the `strftime` documentation.

### Convert textual time and date information back

The ISO C standard does not specify any functions which can convert the output of the `strftime` function back into a binary format. This led to a variety of more-or-less successful implementations with different interfaces over the years. Then the Unix standard was extended by the addition of two functions: `strptime` and `getdate`. Both have strange interfaces but at least they are widely available.

#### Interpret string according to given format

he first function is rather low-level. It is nevertheless frequently used in software since it is better known. Its interface and implementation are heavily influenced by the `getdate` function, which is defined and implemented in terms of calls to `strptime`.

Function: char * strptime (const char *s, const char *fmt, struct tm *tp)
The `strptime` function parses the input string s according to the format string fmt and stores its results in the structure tp.

The input string could be generated by a `strftime` call or obtained any other way. It does not need to be in a human-recognizable format; e.g. a date passed as `"02:1999:9"` is acceptable, even though it is ambiguous without context. As long as the format string fmt matches the input string the function will succeed.

The format string consists of the same components as the format string of the `strftime` function. The only difference is that the flags `_`, `-`, `0`, and `^` are not allowed. Several of the distinct formats of `strftime` do the same work in `strptime` since differences like case of the input do not matter. For reasons of symmetry all formats are supported, though.

The modifiers `E` and `O` are also allowed everywhere the `strftime` function allows them.

The formats are:

`%a`
`%A`
The weekday name according to the current locale, in abbreviated form or the full name.
`%b`
`%B`
`%h`
The month name according to the current locale, in abbreviated form or the full name.
`%c`
The date and time representation for the current locale.
`%Ec`
Like `%c` but the locale's alternative date and time format is used.
`%C`
The century of the year. It makes sense to use this format only if the format string also contains the `%y` format.
`%EC`
The locale's representation of the period. Unlike `%C` it sometimes makes sense to use this format since some cultures represent years relative to the beginning of eras instead of using the Gregorian years.
`%d`
`%e`
The day of the month as a decimal number (range `1` through `31`). Leading zeroes are permitted but not required.
`%Od`
`%Oe`
Same as `%d` but using the locale's alternative numeric symbols. Leading zeroes are permitted but not required.
`%D`
Equivalent to `%m/%d/%y`.
`%F`
Equivalent to `%Y-%m-%d`, which is the ISO 8601 date format. This is a GNU extension following an ISO C99 extension to `strftime`.
`%g`
The year corresponding to the ISO week number, but without the century (range `00` through `99`). Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set. This format is a GNU extension following a GNU extension of `strftime`.
`%G`
The year corresponding to the ISO week number. Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set. This format is a GNU extension following a GNU extension of `strftime`.
`%H`
`%k`
The hour as a decimal number, using a 24-hour clock (range `00` through `23`). `%k` is a GNU extension following a GNU extension of `strftime`.
`%OH`
Same as `%H` but using the locale's alternative numeric symbols.
`%I`
`%l`
The hour as a decimal number, using a 12-hour clock (range `01` through `12`). `%l` is a GNU extension following a GNU extension of `strftime`.
`%OI`
Same as `%I` but using the locale's alternative numeric symbols.
`%j`
The day of the year as a decimal number (range `1` through `366`). Leading zeroes are permitted but not required.
`%m`
The month as a decimal number (range `1` through `12`). Leading zeroes are permitted but not required.
`%Om`
Same as `%m` but using the locale's alternative numeric symbols.
`%M`
The minute as a decimal number (range `0` through `59`). Leading zeroes are permitted but not required.
`%OM`
Same as `%M` but using the locale's alternative numeric symbols.
`%n`
`%t`
Matches any white space.
`%p`
`%P`
The locale-dependent equivalent to `AM' or `PM'. This format is not useful unless `%I` or `%l` is also used. Another complication is that the locale might not define these values at all and therefore the conversion fails. `%P` is a GNU extension following a GNU extension to `strftime`.
`%r`
The complete time using the AM/PM format of the current locale. A complication is that the locale might not define this format at all and therefore the conversion fails.
`%R`
The hour and minute in decimal numbers using the format `%H:%M`. `%R` is a GNU extension following a GNU extension to `strftime`.
`%s`
The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available. `%s` is a GNU extension following a GNU extension to `strftime`.
`%S`
The seconds as a decimal number (range `0` through `60`). Leading zeroes are permitted but not required. Note: The Unix specification says the upper bound on this value is `61`, a result of a decision to allow double leap seconds. You will not see the value `61` because no minute has more than one leap second, but the myth persists.
`%OS`
Same as `%S` but using the locale's alternative numeric symbols.
`%T`
Equivalent to the use of `%H:%M:%S` in this place.
`%u`
The day of the week as a decimal number (range `1` through `7`), Monday being `1`. Leading zeroes are permitted but not required. Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
`%U`
The week number of the current year as a decimal number (range `0` through `53`). Leading zeroes are permitted but not required.
`%OU`
Same as `%U` but using the locale's alternative numeric symbols.
`%V`
The ISO 8601:1988 week number as a decimal number (range `1` through `53`). Leading zeroes are permitted but not required. Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
`%w`
The day of the week as a decimal number (range `0` through `6`), Sunday being `0`. Leading zeroes are permitted but not required. Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
`%Ow`
Same as `%w` but using the locale's alternative numeric symbols.
`%W`
The week number of the current year as a decimal number (range `0` through `53`). Leading zeroes are permitted but not required. Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
`%OW`
Same as `%W` but using the locale's alternative numeric symbols.
`%x`
The date using the locale's date format.
`%Ex`
Like `%x` but the locale's alternative data representation is used.
`%X`
The time using the locale's time format.
`%EX`
Like `%X` but the locale's alternative time representation is used.
`%y`
The year without a century as a decimal number (range `0` through `99`). Leading zeroes are permitted but not required. Note that it is questionable to use this format without the `%C` format. The `strptime` function does regard input values in the range @math{68} to @math{99} as the years @math{1969} to @math{1999} and the values @math{0} to @math{68} as the years @math{2000} to @math{2068}. But maybe this heuristic fails for some input data. Therefore it is best to avoid `%y` completely and use `%Y` instead.
`%Ey`
The offset from `%EC` in the locale's alternative representation.
`%Oy`
The offset of the year (from `%C`) using the locale's alternative numeric symbols.
`%Y`
The year as a decimal number, using the Gregorian calendar.
`%EY`
The full alternative year representation.
`%z`
Equivalent to the use of `%a, %d %b %Y %H:%M:%S %z` in this place. This is the full ISO 8601 date and time format.
`%Z`
The timezone name. Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
`%%`
A literal `%' character.

All other characters in the format string must have a matching character in the input string. Exceptions are white spaces in the input string which can match zero or more white space characters in the format string.

The `strptime` function processes the input string from right to left. Each of the three possible input elements (white space, literal, or format) are handled one after the other. If the input cannot be matched to the format string the function stops. The remainder of the format and input strings are not processed.

The function returns a pointer to the first character it was unable to process. If the input string contains more characters than required by the format string the return value points right after the last consumed input character. If the whole input string is consumed the return value points to the `NULL` byte at the end of the string. If an error occurs, i.e. `strptime` fails to match all of the format string, the function returns `NULL`.

The specification of the function in the XPG standard is rather vague, leaving out a few important pieces of information. Most importantly, it does not specify what happens to those elements of tm which are not directly initialized by the different formats. The implementations on different Unix systems vary here.

The GNU libc implementation does not touch those fields which are not directly initialized. Exceptions are the `tm_wday` and `tm_yday` elements, which are recomputed if any of the year, month, or date elements changed. This has two implications:

• Before calling the `strptime` function for a new input string, you should prepare the tm structure you pass. Normally this will mean initializing all values are to zero. Alternatively, you can set all fields to values like `INT_MAX`, allowing you to determine which elements were set by the function call. Zero does not work here since it is a valid value for many of the fields. Careful initialization is necessary if you want to find out whether a certain field in tm was initialized by the function call.
• You can construct a `struct tm` value with several consecutive `strptime` calls. A useful application of this is e.g. the parsing of two separate strings, one containing date information and the other time information. By parsing one after the other without clearing the structure in-between, you can construct a complete broken-down time.

The following example shows a function which parses a string which is contains the date information in either US style or ISO 8601 form:

```const char *
parse_date (const char *input, struct tm *tm)
{
const char *cp;

/* First clear the result structure.  */
memset (tm, '\0', sizeof (*tm));

/* Try the ISO format first.  */
cp = strptime (input, "%F", tm);
if (cp == NULL)
{
/* Does not match.  Try the US form.  */
cp = strptime (input, "%D", tm);
}

return cp;
}
```

#### A More User-friendly Way to Parse Times and Dates

The Unix standard defines another function for parsing date strings. The interface is weird, but if the function happens to suit your application it is just fine. It is problematic to use this function in multi-threaded programs or libraries, since it returns a pointer to a static variable, and uses a global variable and global state (an environment variable).

Variable: getdate_err
This variable of type `int` contains the error code of the last unsuccessful call to `getdate`. Defined values are:

@math{1}
The environment variable `DATEMSK` is not defined or null.
@math{2}
The template file denoted by the `DATEMSK` environment variable cannot be opened.
@math{3}
Information about the template file cannot retrieved.
@math{4}
The template file is not a regular file.
@math{5}
An I/O error occurred while reading the template file.
@math{6}
Not enough memory available to execute the function.
@math{7}
The template file contains no matching template.
@math{8}
The input date is invalid, but would match a template otherwise. This includes dates like February 31st, and dates which cannot be represented in a `time_t` variable.

Function: struct tm * getdate (const char *string)
The interface to `getdate` is the simplest possible for a function to parse a string and return the value. string is the input string and the result is returned in a statically-allocated variable.

The details about how the string is processed are hidden from the user. In fact, they can be outside the control of the program. Which formats are recognized is controlled by the file named by the environment variable `DATEMSK`. This file should contain lines of valid format strings which could be passed to `strptime`.

The `getdate` function reads these format strings one after the other and tries to match the input string. The first line which completely matches the input string is used.

Elements not initialized through the format string retain the values present at the time of the `getdate` function call.

The formats recognized by `getdate` are the same as for `strptime`. See above for an explanation. There are only a few extensions to the `strptime` behavior:

• If the `%Z` format is given the broken-down time is based on the current time of the timezone matched, not of the current timezone of the runtime environment. Note: This is not implemented (currently). The problem is that timezone names are not unique. If a fixed timezone is assumed for a given string (say `EST` meaning US East Coast time), then uses for countries other than the USA will fail. So far we have found no good solution to this.
• If only the weekday is specified the selected day depends on the current date. If the current weekday is greater or equal to the `tm_wday` value the current week's day is chosen, otherwise the day next week is chosen.
• A similar heuristic is used when only the month is given and not the year. If the month is greater than or equal to the current month, then the current year is used. Otherwise it wraps to next year. The first day of the month is assumed if one is not explicitly specified.
• The current hour, minute, and second are used if the appropriate value is not set through the format.
• If no date is given tomorrow's date is used if the time is smaller than the current time. Otherwise today's date is taken.

It should be noted that the format in the template file need not only contain format elements. The following is a list of possible format strings (taken from the Unix standard):

```%m
%A %B %d, %Y %H:%M:%S
%A
%B
%m/%d/%y %I %p
%d,%m,%Y %H:%M
at %A the %dst of %B in %Y
run job at %I %p,%B %dnd
%A den %d. %B %Y %H.%M Uhr
```

As you can see, the template list can contain very specific strings like `run job at %I %p,%B %dnd`. Using the above list of templates and assuming the current time is Mon Sep 22 12:19:47 EDT 1986 we can obtain the following results for the given input.

@multitable {xxxxxxxxxxxx} {xxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx}

• Input @tab Match @tab Result
• Mon @tab %a @tab Mon Sep 22 12:19:47 EDT 1986
• Sun @tab %a @tab Sun Sep 28 12:19:47 EDT 1986
• Fri @tab %a @tab Fri Sep 26 12:19:47 EDT 1986
• September @tab %B @tab Mon Sep 1 12:19:47 EDT 1986
• January @tab %B @tab Thu Jan 1 12:19:47 EST 1987
• December @tab %B @tab Mon Dec 1 12:19:47 EST 1986
• Sep Mon @tab %b %a @tab Mon Sep 1 12:19:47 EDT 1986
• Jan Fri @tab %b %a @tab Fri Jan 2 12:19:47 EST 1987
• Dec Mon @tab %b %a @tab Mon Dec 1 12:19:47 EST 1986
• Jan Wed 1989 @tab %b %a %Y @tab Wed Jan 4 12:19:47 EST 1989
• Fri 9 @tab %a %H @tab Fri Sep 26 09:00:00 EDT 1986
• Feb 10:30 @tab %b %H:%S @tab Sun Feb 1 10:00:30 EST 1987
• 10:30 @tab %H:%M @tab Tue Sep 23 10:30:00 EDT 1986
• 13:30 @tab %H:%M @tab Mon Sep 22 13:30:00 EDT 1986 The return value of the function is a pointer to a static variable of type `struct tm`, or a null pointer if an error occurred. The result is only valid until the next `getdate` call, making this function unusable in multi-threaded applications. The `errno` variable is not changed. Error conditions are stored in the global variable `getdate_err`. See the description above for a list of the possible error values. Warning: The `getdate` function should never be used in SUID-programs. The reason is obvious: using the `DATEMSK` environment variable you can get the function to open any arbitrary file and chances are high that with some bogus input (such as a binary file) the program will crash.
• Function: int getdate_r (const char *string, struct tm *tp)
The `getdate_r` function is the reentrant counterpart of `getdate`. It does not use the global variable `getdate_err` to signal an error, but instead returns an error code. The same error codes as described in the `getdate_err` documentation above are used, with 0 meaning success. Moreover, `getdate_r` stores the broken-down time in the variable of type `struct tm` pointed to by the second argument, rather than in a static variable. This function is not defined in the Unix standard. Nevertheless it is available on some other Unix systems as well. The warning against using `getdate` in SUID-programs applies to `getdate_r` as well.

### Specifying the Time Zone with `TZ`

In POSIX systems, a user can specify the time zone by means of the `TZ` environment variable. For information about how to set environment variables, see section Environment Variables. The functions for accessing the time zone are declared in `time.h'.

You should not normally need to set `TZ`. If the system is configured properly, the default time zone will be correct. You might set `TZ` if you are using a computer over a network from a different time zone, and would like times reported to you in the time zone local to you, rather than what is local to the computer.

In POSIX.1 systems the value of the `TZ` variable can be in one of three formats. With the GNU C library, the most common format is the last one, which can specify a selection from a large database of time zone information for many regions of the world. The first two formats are used to describe the time zone information directly, which is both more cumbersome and less precise. But the POSIX.1 standard only specifies the details of the first two formats, so it is good to be familiar with them in case you come across a POSIX.1 system that doesn't support a time zone information database.

The first format is used when there is no Daylight Saving Time (or summer time) in the local time zone:

```std offset
```

The std string specifies the name of the time zone. It must be three or more characters long and must not contain a leading colon, embedded digits, commas, nor plus and minus signs. There is no space character separating the time zone name from the offset, so these restrictions are necessary to parse the specification correctly.

The offset specifies the time value you must add to the local time to get a Coordinated Universal Time value. It has syntax like [`+`|`-`]hh[`:`mm[`:`ss]]. This is positive if the local time zone is west of the Prime Meridian and negative if it is east. The hour must be between `0` and `23`, and the minute and seconds between `0` and `59`.

For example, here is how we would specify Eastern Standard Time, but without any Daylight Saving Time alternative:

```EST+5
```

The second format is used when there is Daylight Saving Time:

```std offset dst [offset]`,`start[`/`time]`,`end[`/`time]
```

The initial std and offset specify the standard time zone, as described above. The dst string and offset specify the name and offset for the corresponding Daylight Saving Time zone; if the offset is omitted, it defaults to one hour ahead of standard time.

The remainder of the specification describes when Daylight Saving Time is in effect. The start field is when Daylight Saving Time goes into effect and the end field is when the change is made back to standard time. The following formats are recognized for these fields:

`Jn`
This specifies the Julian day, with n between `1` and `365`. February 29 is never counted, even in leap years.
`n`
This specifies the Julian day, with n between `0` and `365`. February 29 is counted in leap years.
`Mm.w.d`
This specifies day d of week w of month m. The day d must be between `0` (Sunday) and `6`. The week w must be between `1` and `5`; week `1` is the first week in which day d occurs, and week `5` specifies the last d day in the month. The month m should be between `1` and `12`.

The time fields specify when, in the local time currently in effect, the change to the other time occurs. If omitted, the default is `02:00:00`.

For example, here is how you would specify the Eastern time zone in the United States, including the appropriate Daylight Saving Time and its dates of applicability. The normal offset from UTC is 5 hours; since this is west of the prime meridian, the sign is positive. Summer time begins on the first Sunday in April at 2:00am, and ends on the last Sunday in October at 2:00am.

```EST+5EDT,M4.1.0/2,M10.5.0/2
```

The schedule of Daylight Saving Time in any particular jurisdiction has changed over the years. To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then. However, this format has no facilities to let you specify how the schedule has changed from year to year. The most you can do is specify one particular schedule--usually the present day schedule--and this is used to convert any date, no matter when. For precise time zone specifications, it is best to use the time zone information database (see below).

The third format looks like this:

```:characters
```

Each operating system interprets this format differently; in the GNU C library, characters is the name of a file which describes the time zone.

If the `TZ` environment variable does not have a value, the operation chooses a time zone by default. In the GNU C library, the default time zone is like the specification `TZ=:/etc/localtime' (or `TZ=:/usr/local/etc/localtime', depending on how GNU C library was configured; see section Installing the GNU C Library). Other C libraries use their own rule for choosing the default time zone, so there is little we can say about them.

If characters begins with a slash, it is an absolute file name; otherwise the library looks for the file `/share/lib/zoneinfo/characters'. The `zoneinfo' directory contains data files describing local time zones in many different parts of the world. The names represent major cities, with subdirectories for geographical areas; for example, `America/New_York', `Europe/London', `Asia/Hong_Kong'. These data files are installed by the system administrator, who also sets `/etc/localtime' to point to the data file for the local time zone. The GNU C library comes with a large database of time zone information for most regions of the world, which is maintained by a community of volunteers and put in the public domain.

### Functions and Variables for Time Zones

Variable: char * tzname [2]
The array `tzname` contains two strings, which are the standard names of the pair of time zones (standard and Daylight Saving) that the user has selected. `tzname[0]` is the name of the standard time zone (for example, `"EST"`), and `tzname[1]` is the name for the time zone when Daylight Saving Time is in use (for example, `"EDT"`). These correspond to the std and dst strings (respectively) from the `TZ` environment variable. If Daylight Saving Time is never used, `tzname[1]` is the empty string.

The `tzname` array is initialized from the `TZ` environment variable whenever `tzset`, `ctime`, `strftime`, `mktime`, or `localtime` is called. If multiple abbreviations have been used (e.g. `"EWT"` and `"EDT"` for U.S. Eastern War Time and Eastern Daylight Time), the array contains the most recent abbreviation.

The `tzname` array is required for POSIX.1 compatibility, but in GNU programs it is better to use the `tm_zone` member of the broken-down time structure, since `tm_zone` reports the correct abbreviation even when it is not the latest one.

Though the strings are declared as `char *` the user must refrain from modifying these strings. Modifying the strings will almost certainly lead to trouble.

Function: void tzset (void)
The `tzset` function initializes the `tzname` variable from the value of the `TZ` environment variable. It is not usually necessary for your program to call this function, because it is called automatically when you use the other time conversion functions that depend on the time zone.

The following variables are defined for compatibility with System V Unix. Like `tzname`, these variables are set by calling `tzset` or the other time conversion functions.

Variable: long int timezone
This contains the difference between UTC and the latest local standard time, in seconds west of UTC. For example, in the U.S. Eastern time zone, the value is `5*60*60`. Unlike the `tm_gmtoff` member of the broken-down time structure, this value is not adjusted for daylight saving, and its sign is reversed. In GNU programs it is better to use `tm_gmtoff`, since it contains the correct offset even when it is not the latest one.

Variable: int daylight
This variable has a nonzero value if Daylight Saving Time rules apply. A nonzero value does not necessarily mean that Daylight Saving Time is now in effect; it means only that Daylight Saving Time is sometimes in effect.

### Time Functions Example

Here is an example program showing the use of some of the calendar time functions.

```#include <time.h>
#include <stdio.h>

#define SIZE 256

int
main (void)
{
char buffer[SIZE];
time_t curtime;
struct tm *loctime;

/* Get the current time. */
curtime = time (NULL);

/* Convert it to local time representation. */
loctime = localtime (&curtime);

/* Print out the date and time in the standard format. */
fputs (asctime (loctime), stdout);

/* Print it out in a nice format. */
strftime (buffer, SIZE, "Today is %A, %B %d.\n", loctime);
fputs (buffer, stdout);
strftime (buffer, SIZE, "The time is %I:%M %p.\n", loctime);
fputs (buffer, stdout);

return 0;
}
```

It produces output like this:

```Wed Jul 31 13:02:36 1991
Today is Wednesday, July 31.
The time is 01:02 PM.
```

## Setting an Alarm

The `alarm` and `setitimer` functions provide a mechanism for a process to interrupt itself in the future. They do this by setting a timer; when the timer expires, the process receives a signal.

Each process has three independent interval timers available:

• A real-time timer that counts elapsed time. This timer sends a `SIGALRM` signal to the process when it expires.
• A virtual timer that counts processor time used by the process. This timer sends a `SIGVTALRM` signal to the process when it expires.
• A profiling timer that counts both processor time used by the process, and processor time spent in system calls on behalf of the process. This timer sends a `SIGPROF` signal to the process when it expires. This timer is useful for profiling in interpreters. The interval timer mechanism does not have the fine granularity necessary for profiling native code.

You can only have one timer of each kind set at any given time. If you set a timer that has not yet expired, that timer is simply reset to the new value.

You should establish a handler for the appropriate alarm signal using `signal` or `sigaction` before issuing a call to `setitimer` or `alarm`. Otherwise, an unusual chain of events could cause the timer to expire before your program establishes the handler. In this case it would be terminated, since termination is the default action for the alarm signals. See section Signal Handling.

The `setitimer` function is the primary means for setting an alarm. This facility is declared in the header file `sys/time.h'. The `alarm` function, declared in `unistd.h', provides a somewhat simpler interface for setting the real-time timer.

Data Type: struct itimerval
This structure is used to specify when a timer should expire. It contains the following members:
`struct timeval it_interval`
This is the period between successive timer interrupts. If zero, the alarm will only be sent once.
`struct timeval it_value`
This is the period between now and the first timer interrupt. If zero, the alarm is disabled.

The `struct timeval` data type is described in section Elapsed Time.

Function: int setitimer (int which, struct itimerval *new, struct itimerval *old)
The `setitimer` function sets the timer specified by which according to new. The which argument can have a value of `ITIMER_REAL`, `ITIMER_VIRTUAL`, or `ITIMER_PROF`.

If old is not a null pointer, `setitimer` returns information about any previous unexpired timer of the same kind in the structure it points to.

The return value is `0` on success and `-1` on failure. The following `errno` error conditions are defined for this function:

`EINVAL`
The timer period is too large.

Function: int getitimer (int which, struct itimerval *old)
The `getitimer` function stores information about the timer specified by which in the structure pointed at by old.

The return value and error conditions are the same as for `setitimer`.

`ITIMER_REAL`
This constant can be used as the which argument to the `setitimer` and `getitimer` functions to specify the real-time timer.
`ITIMER_VIRTUAL`
This constant can be used as the which argument to the `setitimer` and `getitimer` functions to specify the virtual timer.
`ITIMER_PROF`
This constant can be used as the which argument to the `setitimer` and `getitimer` functions to specify the profiling timer.

Function: unsigned int alarm (unsigned int seconds)
The `alarm` function sets the real-time timer to expire in seconds seconds. If you want to cancel any existing alarm, you can do this by calling `alarm` with a seconds argument of zero.

The return value indicates how many seconds remain before the previous alarm would have been sent. If there is no previous alarm, `alarm` returns zero.

The `alarm` function could be defined in terms of `setitimer` like this:

```unsigned int
alarm (unsigned int seconds)
{
struct itimerval old, new;
new.it_interval.tv_usec = 0;
new.it_interval.tv_sec = 0;
new.it_value.tv_usec = 0;
new.it_value.tv_sec = (long int) seconds;
if (setitimer (ITIMER_REAL, &new, &old) < 0)
return 0;
else
return old.it_value.tv_sec;
}
```

There is an example showing the use of the `alarm` function in section Signal Handlers that Return.

If you simply want your process to wait for a given number of seconds, you should use the `sleep` function. See section Sleeping.

You shouldn't count on the signal arriving precisely when the timer expires. In a multiprocessing environment there is typically some amount of delay involved.

Portability Note: The `setitimer` and `getitimer` functions are derived from BSD Unix, while the `alarm` function is specified by the POSIX.1 standard. `setitimer` is more powerful than `alarm`, but `alarm` is more widely used.

## Sleeping

The function `sleep` gives a simple way to make the program wait for a short interval. If your program doesn't use signals (except to terminate), then you can expect `sleep` to wait reliably throughout the specified interval. Otherwise, `sleep` can return sooner if a signal arrives; if you want to wait for a given interval regardless of signals, use `select` (see section Waiting for Input or Output) and don't specify any descriptors to wait for.

Function: unsigned int sleep (unsigned int seconds)
The `sleep` function waits for seconds or until a signal is delivered, whichever happens first.

If `sleep` function returns because the requested interval is over, it returns a value of zero. If it returns because of delivery of a signal, its return value is the remaining time in the sleep interval.

The `sleep` function is declared in `unistd.h'.

Resist the temptation to implement a sleep for a fixed amount of time by using the return value of `sleep`, when nonzero, to call `sleep` again. This will work with a certain amount of accuracy as long as signals arrive infrequently. But each signal can cause the eventual wakeup time to be off by an additional second or so. Suppose a few signals happen to arrive in rapid succession by bad luck--there is no limit on how much this could shorten or lengthen the wait.

Instead, compute the calendar time at which the program should stop waiting, and keep trying to wait until that calendar time. This won't be off by more than a second. With just a little more work, you can use `select` and make the waiting period quite accurate. (Of course, heavy system load can cause additional unavoidable delays--unless the machine is dedicated to one application, there is no way you can avoid this.)

On some systems, `sleep` can do strange things if your program uses `SIGALRM` explicitly. Even if `SIGALRM` signals are being ignored or blocked when `sleep` is called, `sleep` might return prematurely on delivery of a `SIGALRM` signal. If you have established a handler for `SIGALRM` signals and a `SIGALRM` signal is delivered while the process is sleeping, the action taken might be just to cause `sleep` to return instead of invoking your handler. And, if `sleep` is interrupted by delivery of a signal whose handler requests an alarm or alters the handling of `SIGALRM`, this handler and `sleep` will interfere.

On the GNU system, it is safe to use `sleep` and `SIGALRM` in the same program, because `sleep` does not work by means of `SIGALRM`.

Function: int nanosleep (const struct timespec *requested_time, struct timespec *remaining)
If resolution to seconds is not enough the `nanosleep` function can be used. As the name suggests the sleep interval can be specified in nanoseconds. The actual elapsed time of the sleep interval might be longer since the system rounds the elapsed time you request up to the next integer multiple of the actual resolution the system can deliver.

*`requested_time` is the elapsed time of the interval you want to sleep.

The function returns as *`remaining` the elapsed time left in the interval for which you requested to sleep. If the interval completed without getting interrupted by a signal, this is zero.

`struct timespec` is described in See section Elapsed Time.

If the function returns because the interval is over the return value is zero. If the function returns @math{-1} the global variable errno is set to the following values:

`EINTR`
The call was interrupted because a signal was delivered to the thread. If the remaining parameter is not the null pointer the structure pointed to by remaining is updated to contain the remaining elapsed time.
`EINVAL`
The nanosecond value in the requested_time parameter contains an illegal value. Either the value is negative or greater than or equal to 1000 million.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `nanosleep` is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `nanosleep` should be protected using cancellation handlers.

The `nanosleep` function is declared in `time.h'.