27. Date input formats

First, a quote:

Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done better than handing down our present system. It is like a set of trapezoidal building blocks, with no vertical or horizontal surfaces, like a language in which the simplest thought demands ornate constructions, useless particles and lengthy circumlocutions. Unlike the more successful patterns of language and science, which enable us to face experience boldly or at least level-headedly, our system of temporal calculation silently and persistently encourages our terror of time.

... It is as though architects had to measure length in feet, width in meters and height in ells; as though basic instruction manuals demanded a knowledge of five different languages. It is no wonder then that we often look into our own immediate past or future, last Tuesday or a week from Sunday, with feelings of helpless confusion. ...

--- Robert Grudin, Time and the Art of Living.

This section describes the textual date representations that GNU programs accept. These are the strings you, as a user, can supply as arguments to the various programs. The C interface (via the getdate function) is not described here.

Although the date syntax here can represent any possible time since the year zero, computer integers often cannot represent such a wide range of time. On POSIX systems, the clock starts at 1970-01-01 00:00:00 UTC: POSIX does not require support for times before the POSIX Epoch and times far in the future. Traditional Unix systems have 32-bit signed time_t and can represent times from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 UTC. Systems with 64-bit signed time_t can represent all the times in the known lifetime of the universe.

27.1 General date syntax		Common rules.
27.2 Calendar date items		19 Dec 1994.
27.3 Time of day items		9:20pm.
27.4 Time zone items		EST, PDT, GMT, ...
27.5 Day of week items		Monday and others.
27.6 Relative items in date strings		next tuesday, 2 years ago.
27.7 Pure numbers in date strings		19931219, 1440.
27.8 Authors of getdate		Bellovin, Eggert, Salz, Berets, et al.