day-of-week

Arguments: ut

The value returned by day-of-week and related functions depends on the time zone where the machine running Lisp is located and also on whether daylight saving time is in effect. See the discussion in Day and date calculation functions and their relation to time zones in miscellaneous.htm for examples of how time zone and daylight saving time can affect results.

This function performs calculations on the day/date portion of a universal time, that is the decoded universal time ignoring the second, minute, and hour values (see decode-universal-time).

day-of-week returns the number corresponding to the day of the week of the universal time ut (which must be a non-negative integer). The value is the same as the seventh return value of decode-universal-time when the optional time-zone argument is not specified. Monday is day 0, Tuesday is day 1, and so on through Sunday, day 6.

The values of the constants *decoded-time-monday*, *decoded-time-tuesday*, *decoded-time-wednesday*, *decoded-time-thursday*, *decoded-time-friday*, *decoded-time-saturday*, and *decoded-time-sunday* are the integers corresponding to the named day of the week.

;; Results depend on the timezone value and whether daylight saving
;; time is in effect. These are not settable by this function and
;; the values are inherited from the Operating system. To see them,
;; evaluate (decode-universal-time (get-universal-time)):

cl-user(29): (decode-universal-time (get-universal-time))
RETURNS 11 38 14 5 12 2017 1 nil 8

;; That form was evaluated at 2:38:11 PM (14:38:11) on Tuesday,
;; December 5 at Franz Inc. HQ in Oakland, California. The eighth
;; return value (nil) indicates daylight saving time was not in effect.
;; The ninth return value (8) means the timezone was 8, 8 hours
;; earlier than Greenwich Mean Time (GMT). If you are in a different
;; location or the date is different, you will get different values.

cl-user(101): (setq ut (encode-universal-time 0 0 12 4 5 2017))
3702913200
cl-user(102): (day-of-week ut)
3
cl-user(103): (decode-universal-time ut)
0 0 12 4 5 2017 3 t 8 ;; same as the seventh value (3)
cl-user(106): 

See Day and date calculation functions and their relation to time zones in miscellaneous.htm for more information on this and related functions.

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This page is new in the 10.1 release.
Created 2019.8.20.