It seems to me I can either set a time for today or some relative time in the future.

But can I combine the two somehow to set a timer for 7am tomorrow, for example?


Yes - call run-at-time using the encode-time method to pass the time argument as mentioned in its doc string:

(run-at-time TIME REPEAT FUNCTION &rest ARGS)

  Probably introduced at or before Emacs version 19.26.

Perform an action at time TIME.
Repeat the action every REPEAT seconds, if REPEAT is non-nil.
REPEAT may be an integer or floating point number.
TIME should be one of:
- a string giving today's time like "11:23pm"
  (the acceptable formats are HHMM, H:MM, HH:MM, HHam, HHAM,
  HHpm, HHPM, HH:MMam, HH:MMAM, HH:MMpm, or HH:MMPM;
  a period `.' can be used instead of a colon `:' to separate
  the hour and minute parts);
- a string giving a relative time like "90" or "2 hours 35 minutes"
  (the acceptable forms are a number of seconds without units
  or some combination of values using units in `timer-duration-words');
- nil, meaning now;
- a number of seconds from now;
- a value from `encode-time';
- or t (with non-nil REPEAT) meaning the next integral
  multiple of REPEAT.

The action is to call FUNCTION with arguments ARGS.

This function returns a timer object which you can use in

So you can say

(run-at-time (encode-time '(0 0 7 13 6 2020 nil t nil)) nil #+my-func)

to run my-func once at 7am EDT (local time) on 2020-06-13.

N.B. For versions of emacs < 27 (?), encode-time had a different signature, so the above needs to be modified like this:

(run-at-time (apply #'encode-time '(0 0 7 13 6 2020 nil t nil)) nil #+my-func)

Calculating tomorrow from today is not difficult (there may be better ways to do that than the one I use below which was thrown together in a hurry):

(defun tomorrow ()
  ;;; the `(nthcdr 3 ...)' gets rid of the SECOND, MINUTE, HOUR values
  (let ((now-plus-1-day (nthcdr 3 (decode-time (+ (* 24 60 60) 
                                                  (time-to-seconds (current-time)))))))
    ;;; now keep three entries and get rid of the DST,TIMEZONE entries
    (setf (nthcdr 3 now-plus-1-day) nil)
    ;;; return (MONTH DAY YEAR)

This returns a three-element list (MONTH DAY YEAR).

So now you can combine the two and say

(run-at-time (encode-time `(0 0 7 ,@(tomorrow) nil t nil)) nil #+my-func)

using the backquote mechanism to splice in the list that tomorrow returns.

N.B. For versions of emacs < 27 (?), encode-time had a different signature, so the above needs to be modified like this:

(run-at-time (apply #'encode-time '(0 0 7 ,@(tomorrow) nil t nil)) nil #+my-func)

You should check the doc string of encode-time as well (the following is from bleeding-edge emacs 28.0.50 - as noted above, for emacs versions < 27(?), the signature of encode-time was different, so check the doc string locally):

encode-time is a built-in function in `C source code'.


  Probably introduced at or before Emacs version 19.29.
  This function does not change global state, including the match data.

Convert TIME to a timestamp.

in the style of `decode-time', so that (encode-time (decode-time ...)) works.
In this list, ZONE can be nil for Emacs local time, t for Universal
Time, `wall' for system wall clock time, or a string as in the TZ
environment variable.  It can also be a list (as from
`current-time-zone') or an integer (as from `decode-time') applied
without consideration for daylight saving time.  If ZONE specifies a
time zone with daylight-saving transitions, DST is t for daylight
saving time, nil for standard time, and -1 to cause the daylight
saving flag to be guessed.

As an obsolescent calling convention, if this function is called with
6 or more arguments, the first 6 arguments are SECOND, MINUTE, HOUR,
DAY, MONTH, and YEAR, and specify the components of a decoded time,
where DST assumed to be -1 and FORM is omitted.  If there are more
than 6 arguments the *last* argument is used as ZONE and any other
extra arguments are ignored, so that (apply #'encode-time
(decode-time ...)) works.  In this obsolescent convention, DST and
ZONE default to -1 and nil respectively.

Years before 1970 are not guaranteed to work.  On some systems,
year values as low as 1901 do work.

EDIT: added some backward-compatibiity notes for encode-time. I'm not sure when the signature changed but a comment indicates that 26.2 uses the old implementation and my experiments with 26.3 indicate the same thing. I have not tested emacs-27, hence the question marks.

Thanks to @Tom for pointing the problem out.

  • Thanks for the detailed answer. I missed encode-time, because I was reading the info manual, and it's only a single sentence there among lots of other info. BTW, it would be more convenient if the user could simply specify "7am 1 day" meaning 7am a day from now. Strange that it's not supported, because the elements are there. – Tom Jun 12 '20 at 17:32
  • That's a UI matter and e.g. org-read-date allows some things like that but not exactly what you want: maybe (probably?) someone has written a function to translate human-friendly specs like "7am tomorrow" to the format that run-at-time expects. If not, maybe you can write your own implementation: if it's good enough, then everybody will adopt it! The main problem will be to detect ambiguity and inform the (human) user so that they can fix it. – NickD Jun 12 '20 at 18:00
  • I use orgmode for scheduling and its date read function is really convenient, so it could probably be adapted for use in run-at-time in some form. Orgmode is part of emacs, so it would make sense to reuse the code, instead of writing it from scratch. – Tom Jun 12 '20 at 18:04
  • But AFAIK it cannot combine times and relative dates: 7am +1d is not understood. There may be other limitations. – NickD Jun 12 '20 at 18:11
  • If it's past 7am then 7am in orgmode gives you 7am tomorrow which is nice. run-at-time could do this by default. And if it's before 7am and you want 7am tomorrow then I usually use the day. So if it's 6am Sat then 7am Sun sets 7am to tomorrow. – Tom Jun 12 '20 at 18:16

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