How to get time difference with Elisp?

[Karl Voit]

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

Can you help me? Thanks!

-- 
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