The basic idea for this is to convert both times into "epoch" seconds. Do some subtraction in seconds. Then convert back to "human hours/min/secs time. Or in your case, you may want just 65 minutes.

Epoch time is an signed integer value in number of seconds that usually starts at Jan 1, 1970, midnight for historical reasons.

Look at Perl built in functions: gmtime, localtime, time. Then look at the module: Time::Local. That module give the inverse functions.

For benchmarking, forget about the year, month, date. Normally just the time expressed in hours , min and seconds is all that is needed. In your case just minutes which is of course is seconds / 60.

updated : modules and functions to look for.