Half a timezone

I always thought timezones had differences of whole hours. Can anyone explain the output below?

<?php
$d = new DateTime();

$d->setTimezone(new DateTimeZone('Europe/London'));
echo "<pre>London\t\t" . $d->format ('H:i:s') . '<br>';

$d->setTimezone(new DateTimeZone('Asia/Hong_Kong'));
echo "Hong Kong\t" . $d->format ('H:i:s') . '<br>';

$d->setTimezone(new DateTimeZone('Asia/Calcutta'));
echo "Calcutta\t" . $d->format ('H:i:s') . '</pre>';

?>

output

London      19:45:35
Hong Kong   02:45:35
Calcutta    00:15:35    (15 minutes ???)