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
Australia/Melbourne: Sat, 23 Dec 1978 06:15:00 +1100