elcron is an emulator of cron for Emacs. It has functionality to schedule function calls at specific time points, following a cron-like expression.

You can call the function elcron-schedule to schedule a function call at specific times. For example, the following function call schedules a message starting with the third second then every five seconds in the 3rd to 10th minute every hour, every day, every month and every year.

(setq elcron-test-timer
      (elcron-schedule "2/5 5-10"
                     (lambda () (message "%s cron fired"
                                         (format-time-string "%T %d/%m/%Y")))))

;; To cancel the elcron timer, simply do
(cancel-timer elcron-test-timer)

A elcron expression can be

• a cron-like string of up to 7 parts, SECONDS MINUTES HOURS DAY MONTH WEEKDAY YEAR. For example, 0/5 14,18,20-39,52 * * JAN,MAR,SEP MON-FRI 2002-2050.
• a vector of up to 7 parts. For example [(/ 0 5) (14 18 (- 20 39) 52) * * (jan mar sep) (- mon fri) (- 2002 2050)].
• a plist. For example (:second (/ 0 5) :minute (14 18 (- 20 39) 52) :month (jan mar sep) :weekday (- mon fri) :year (- 2002 2050)).

For more complicated elcron expressions, see here for the format.

Mostly because I got interested in efficient ways to compute elcron trigger times given an expression. Let me know if you have better methods.

With that being said, elcron is not meant as a replacement for a system’s cron. For one thing, elcron timers will not survive across Emacs sessions, nor will they run when Emacs is not running. Also, elcron uses Emacs timers which are triggered when Emacs is idle and so accurate triggers at specific times are not guaranteed. The elcron scheduler may even skip some triggers if Emacs is busy for the the whole duration between two or more triggers.