How to subtract timestamp in elisp?

[stardiviner]

I'm find an Elisp solution to subtract timestamps like "00:12:35".

I hope a function can subtract two timestamps:

    00:12:35 - 00:10:45 = 00:01:50

Is there some hints or suggestion like function name or Emacs library or 
package?
Or can convert timestamp into another format then convert back to timestamp.

I use those timestamps in "ffmpeg" command video cut clip command option "-ss" 
and "-t".

Or Linux command is acceptable. I can write a function to execute shell command 
then parse the result.

Here is my current code:

#+begin_src emacs-lisp
(defun ffmpeg-cut-clip (input-filename start-timestamp end-timestamp 
output-filename)
  (interactive (list
                (read-file-name "FFmpeg input filename: ")
                (read-string "FFmpeg start timestamp: ")
                (read-string "FFmpeg time timestamp: ")
                (read-file-name "FFmpeg output filename: ")))
  (let ((time-timestamp (FUNC start-timestamp end-timestamp)))
    (make-process
     :name "ffmpeg cut clip"
     ;; "ffmpeg -i input-filename -ss start-timestamp -t time-timestamp -codec 
copy output-filename"
     :command (list "ffmpeg"
                    "-i" input-filename
                    "-ss" start-timestamp
                    "-t" time-timestamp
                    "-codec copy"
                    output-filename)
     :buffer "*ffmpeg-cut-clip*"
     :sentinel (lambda (proc event)
                 (message "FFmpeg cut video clip finished.")))))
#+end_src

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[ stardiviner ]
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