I have had great success finding time/date differences using the DateTime module. Here is an example where I get the difference in time between now and the creation date on a file.

use DateTime;

my $current = DateTime->now;
my $created = DateTime->from_epoch( epoch => (stat $_ )[9] );
my $duration = $current - $created;
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Then to get the actual duration values from $duration take a look at the DateTime::Duration module.