php_unix_time_zone.txt

>>> $dt1 = new DateTime()
=> <DateTime #000000007766f6860000000009e51d7d> {
       date: "2015-02-09 12:13:50",
       timezone_type: 3,
       timezone: "Asia\/Tokyo"
   }
>>> $dt2 = new DateTime('@'.$dt1->getTimestamp())
=> <DateTime #000000007766f6810000000009e51d7d> {
       date: "2015-02-09 03:13:50",
       timezone_type: 1,
       timezone: "+00:00"
   }
>>> $ddt = $dt1->diff($dt2)
=> <DateInterval #000000007766f6840000000009e51ccd> {
       y: 0,
       m: 0,
       d: 0,
       h: 0,
       i: 0,
       s: 0,
       weekday: 0,
       weekday_behavior: 0,
       first_last_day_of: 0,
       invert: 0,
       days: 0,
       special_type: 0,
       special_amount: 0,
       have_weekday_relative: 0,
       have_special_relative: 0
   }
>>> $dt1->format('Y-m-d H:i:s')
=> "2015-02-09 12:13:50"
>>> $dt2->format('Y-m-d H:i:s')
=> "2015-02-09 03:13:50"
>>> $dt2->setTimezone(new DateTimeZone(date_default_timezone_get()));
=> <DateTime #000000007766f6810000000009e51d7d> {
       date: "2015-02-09 12:13:50",
       timezone_type: 3,
       timezone: "Asia\/Tokyo"
   }
>>> $dt2->format('Y-m-d H:i:s')
=> "2015-02-09 12:13:50"