Using the timezone when parsing a string with a date representation:

<?php
  $tz1 = new DateTimeZone("Pacific/Honolulu");
  $tz2 = new DateTimeZone("Australia/Melbourne");

  $ts = new DateTimeImmutable("1978-12-22 09:15", $tz1);
  echo $ts->getTimezone()->getName(), ':    ',
    $ts->format(DateTime::RFC2822), "<br/>";

  $melbourne = $ts->setTimezone($tz2);

  echo $melbourne->getTimezone()->getName(), ': ',
    $melbourne->format(DateTime::RFC2822), "<br/>";
?>
Output
Pacific/Honolulu: Fri, 22 Dec 1978 09:15:00 -1000
Australia/Melbourne: Sat, 23 Dec 1978 06:15:00 +1100