I'd like to calculate duration time between two dates in calc.

<2017-03-21 Mon 18:45> - <2017-03-20 Mon 18:45> = 24 hr

How can I do it?

I can insert a date to calc by typing ' <2017-03-20 18:45> at calc prompt. I can change the date format by typing dd10 to org-format.

But when I subtract <2017-03-20 Mon 18:45> from <2017-03-21 Mon 18:45>, I got 1.000035.

How can i get duration in hours and minutes?

  • 2
    The result is in days. Just multiply by 24 to get hours.
    – Juancho
    Mar 20, 2017 at 15:43
  • 1
    is there easier function to convert numbers to HH:MM:SS format? M-9 t P is closest I found. Mar 21, 2017 at 0:10
  • 1
    Depending on your task, org-duration.el might be helpful here.
    – rasmus
    Mar 22, 2017 at 9:21
  • org-duration.el will do. But I wanted to do it with the calc proper. (to report a bug in org ;-)) Mar 22, 2017 at 9:24

1 Answer 1


It looks like the part that's missing is printing the value on the top of the stack in the desired format. Try this:

(defun my/days-to-time-string (days)
  "Format number of DAYS as a string (HH:MM:SS)."
  (let* ((hours (* days 24))
         (minutes (* (mod hours 1) 60))
         (seconds (* (mod minutes 1) 60)))
    (format "%02d:%02d:%02d" (floor hours) (floor minutes) (floor seconds))))

(defmath days-to-time-string ()
  "Print the value on calc stack as days converted to hours."
  (let ((days (string-to-number (calc-eval 1 'top))))
    (message (my/days-to-time-string days))))

Unless you create a binding you would use it with M-x calc-days-to-time-string.

  • 1
    This is very close to what I want. Unfortunately, it prints 24:59:60 when I give 1.041666. Mar 23, 2017 at 8:29
  • @YasushiShoji Whoops; good catch! Tricky edge cases :-) Fixed ;-) I'm sure you can work something a little more complex out if you care about not rounding seconds down.
    – ebpa
    Mar 25, 2017 at 17:47

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