Looking at Programming Perl, first edition (I can't believe I still have it!), there is a timelocal.pl in the perl4 standard library. So the code would go something like

require "timelocal.pl";

@old_time = ($sec, ..., $isdst);
$old_seconds = &timelocal( @old_time);
$new_seconds = time();

if ( $new_seconds - $old_seconds > 3600 * 24 * 15) {
   print "Interval is greater than 15 days\n";
}
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