elisp timedelta output format

Question

When try to print timedelta as below:

#+BEGIN_SRC elisp :results output
(print (time-subtract
    (parse-time-string "2018-10-20 08:50:36.343")
    (parse-time-string "2018-10-20 08:47:43.029")))
#+END_SRC

#+RESULTS:
: 
: (-7 3 0 0)

The output format looks wired:

1. The seconds is negative.
2. Output format not readable, I wish it's "00:02:53.314".

Answer 1

#'time-subtract's docstring says "See ‘current-time-string’ for the various forms of a time value." #'current-time-string specifies times to be in this format:

If SPECIFIED-TIME is given, it is a time to format instead of the current time. The argument should have the form (HIGH LOW . IGNORED).

However, #'parse-time-string says this:

Parse the time-string STRING into (SEC MIN HOUR DAY MON YEAR DOW DST TZ).

So one shouldn't pass this value into #'time-subtract. But there is #'date-to-time:

ELISP> (date-to-time "2018-10-20 08:50:36.343")
(23499 9372)

And we can pass these values to #'time-subtract:

ELISP> (time-subtract (date-to-time "2018-10-20 08:50:36.343") 
                      (date-to-time "2018-10-20 08:47:43.029"))
(0 173)

This makes sense; 8:50:36 is exactly 173 seconds after 8:47:43! Cool! Note that this throws away the milliseconds in the timestamps. This is a limitation of how Emacs deals with time.

Now let's get it into the output format you want: 00:02:53.314. We can use #'format-time-string to do so:

ELISP> (format-time-string "%H:%M:%S"
                           (time-subtract (date-to-time "2018-10-20 08:50:36.343")
                                          (date-to-time "2018-10-20 08:47:43.029"))
                           t)
"00:02:53"

Note that you have to give the third argument ZONE to #'format-time-string, or it will format it in your local time zone. Passing t there formats it in UTC. This is definitely confusing.

Answer 2

That way it seems to produce something reasonable:

#+BEGIN_SRC elisp :results output
(print (time-subtract
    (parse-iso8601-time-string "2018-10-20 08:50:36")
    (parse-iso8601-time-string "2018-10-20 08:47:43")))
#+END_SRC

#+RESULTS:

: : (0 173)