Even if you extract the hours, minutes, and seconds; you'll still need to convert the hours (you might need to "borrow" 1 from the hours), and convert the seconds to fractional minutes.

In this case, I would simply convert the string to second, subtract, and convert back.

If you are timing something, though, you may wish to look into the various timing modules, or (poor man's method) capture the value of time() at different points in the program, and convert the difference to minutes at the end.