While there are many date & time packages available on CPAN, for my purposes, I generally just store times as returned by time() (seconds since 1970/01/01 00:00:00 IIRC). Then taking the difference is a simple subtraction. Of course, you'll need to format the results when you're done. But I typically do something like:

my $start=time;
# ... long processing job goes here...
my $dur = time - $start;

# Print out in appropriate format:
print "Job took $dur seconds\n";

# For longer jobs, I often use one of:
printf "Job took %5.2 minutes\n", $dur/60.0;
printf "Job took %5.2 hours\n", $dur/(60*60.0);
printf "Job took %5.2 days\n", $dur/(24*60*60.0);
printf "Job took %5.2 weeks\n", $dur/(7*24*60*60.0);
# etc.
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Pretty simple and adequate for what I do. If I cared for more accuracy or features, I'd check out one of the Date modules on CPAN.

...roboticus

I have a fancier formatting module to show in format "Days HH:MM:SS", but the margin is too small for me to note it here. (Actually, it's on my server at home, and I haven't brought MY::Utils.pm into work with me yet. It's on my TODO list.)