public static function date_time_subtract(EE_Base_Class $obj, $dttfield, $what = 'years', $value = 1)
 {
     //get the raw UTC date
     $dtt = $obj->get_raw($dttfield);
     $new_date = self::calc_date($dtt, $what, $value, '-');
     $obj->set($dttfield, $dtt);
     return $obj;
 }