Re: How to get time difference with Elisp?

[<tomas@tuxteam.de>]

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On Tue, Jul 12, 2016 at 12:46:37PM +0200, Karl Voit wrote:
> Hi!
> 
> I need to determine total office hours of a day without the time
> spent in lunch break. The source data is officebegin, officeend,
> lunchbreakbegin, lunchbreakend - all in string format "HH:MM" like
> "14:58".
> 
> So far, I failed miserably to find the right combination of
> parse-time-string, encode-time, time-substract.
> 
> Even determining the difference between only two times resulted in
> errors to me:
> 
> (setq difference (time-subtract (encode-time (parse-time-string "12:24"))
>                                 (encode-time (parse-time-string "11:45"))))
> 
> ... results in: time-subtract: Wrong number of arguments: encode-time, 1

The immediate problem is that parse-time-string returns a list of values,
but encode-time expects separate arguments. Apply takes care of that:

      (apply #'encode-time (parse-time-string "12:24"))

etc.

Now the problem is that for your time string day, month, year turn up
as nil, something encode-time doesn't like at all. I guess you'd have
to fill in missing stuff with the values from current-time. Here's
a rough sketch to start with:

  (let ((time-default (decode-time (current-time))))
    (setq difference
      (time-subtract
        (encode-time
          (mapcar* (lambda (x y) (or x y))
            (apply #'encode-time (parse-time-string "12:24"))
            time-default))
        (encode-time
          (mapcar* (lambda (x y) (or x y))
            (apply #'encode-time (parse-time-string "11:45"))
            time-default))

Of course, there might be functions around which make this much simpler.
If not, I think abstracting away some part as a function might enhance
readability a lot.

Note that I used mapcar* which I think is part of cl.

regards
- -- tomás
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