You may also find DateTime useful:

use DateTime;
my $date = DateTime->new( year   => 2007,
                          month  => 8,
                          day    => 25,
                          hour   => 23,
                          minute => 59,
                          second => 59,
                          time_zone => 'UTC',
                        );



print $date->ymd, "\t", $date->hms, "\n";

$date->add( hours => 36 );

print $date->ymd, "\t", $date->hms, "\n";
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Outputs:

2007-08-25      23:59:59
2007-08-27      11:59:59
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