Попробуйте использовать Дата :: Calc
use Date::Calc qw(Add_Delta_DHMS);
($year2, $month2, $day2, $h2, $m2, $s2) =
Add_Delta_DHMS( $year, $month, $day, $hour, $minute, $second, $days_offset, $hour_offset, $minute_offset, $second_offset );
($y,$m,$d,$H,$M,$S) = Add_Delta_DHMS(Today_and_Now(), 0, 0, -15, 0);