27.1 General date syntax

A date is a string, possibly empty, containing many items separated by whitespace. The whitespace may be omitted when no ambiguity arises. The empty string means the beginning of today (i.e., midnight). Order of the items is immaterial. A date string may contain many flavors of items:

• calendar date items
• time of the day items
• time zone items
• day of the week items
• relative items
• pure numbers.

We describe each of these item types in turn, below.

A few numbers may be written out in words in most contexts. This is most useful for specifying day of the week items or relative items (see below). Here is the list: `first' for 1, `next' for 2, `third' for 3, `fourth' for 4, `fifth' for 5, `sixth' for 6, `seventh' for 7, `eighth' for 8, `ninth' for 9, `tenth' for 10, `eleventh' for 11 and `twelfth' for 12. Also, `last' means exactly -1.

When a month is written this way, it is still considered to be written numerically, instead of being "spelled in full"; this changes the allowed strings.

In the current implementation, only English is supported for words and abbreviations like `AM', `DST', `EST', `first', `January', `Sunday', `tomorrow', and `year'.

The output of date is not always acceptable as a date string, not only because of the language problem, but also because there is no standard meaning for time zone items like `IST'. When using date to generate a date string intended to be parsed later, specify a date format that is independent of language and that does not use time zone items other than `UTC' and `Z'. Here are some ways to do this:

$ LC_ALL=C TZ=UTC0 date Fri Dec 15 19:48:05 UTC 2000 $ TZ=UTC0 date +"%Y-%m-%d %H:%M:%SZ" 2000-12-15 19:48:05Z $ date --iso-8601=seconds # a GNU extension 2000-12-15T11:48:05-0800 $ date --rfc-822 # a GNU extension Fri, 15 Dec 2000 11:48:05 -0800 $ date +"%Y-%m-%d %H:%M:%S %z" # %z is a GNU extension. 2000-12-15 11:48:05 -0800

Alphabetic case is completely ignored in dates. Comments may be introduced between round parentheses, as long as included parentheses are properly nested. Hyphens not followed by a digit are currently ignored. Leading zeros on numbers are ignored.