Elisp: Print Date Time

By Xah Lee. Date: . Last updated: .

ISO date-time format

yyyy-mm-dd

(format-time-string "%F")
;; "2026-02-26"

;; or

(format-time-string "%Y-%m-%d")
;; "2026-02-26"

ISO 8601 Format Extended

;; local time
;; no time zone
(format-time-string "%FT%T")
"2026-02-26T14:41:34"

;; local time
;; with time zone
;; sans colon in time zone
(format-time-string "%FT%T%z")
;; "2026-02-26T14:46:06-0800"

;; local time
;; with time zone
;; with colon in time zone
(concat
 (format-time-string "%Y-%m-%dT%T")
 ((lambda (x) (concat (substring x 0 3) ":" (substring x 3 5)))
  (format-time-string "%z")))
;; "2026-02-26T14:46:13-08:00"

;; UTC.
;; with time zone of Z
(format-time-string "%FT%TZ" nil t)
;; "2026-02-26T22:50:12Z"

;; UTC.
;; with milliseconds.
;; with time zone of Z
(format-time-string "%FT%T.%3NZ" nil t)
;; "2026-02-26T22:51:36.221Z"

Common computer format

(format-time-string "%Y%m%d_%H%M%S")
;; "20260226_130818"

Unix Time Format

(number of seconds since .)

;; unix time
(format-time-string "%s") ; "1291104066"

Names for Month and Week

;; full month name
(format-time-string "%B") ; "November"

;; abbreviated month name
(format-time-string "%b") ; "Nov"
;; full week name
(format-time-string "%A") ; "Tuesday"

;; abbreviated week name
(format-time-string "%a") ; "Tue"

Ordinal Date Format

format-time-string also supports ordinal date format. For example:

(format-time-string "%Y-%j") ; "2010-334" for 2010-11-30

Elisp, Datetime