Setting a default timezone:
<?php
    date_default_timezone_set("Europe/Oslo");
    $ts = new DateTime("1978-12-22 09:15");
    echo $ts->format("e");
?>

Getting a default timezone:
<?php
    $default_identifier = date_default_timezone_get();
    echo $default_identifier;
?>

Default timezone is 'guessed' in the following order: