If you take a look at the date reference, a lower-case h
is the 12-hour formatted hour. You need a capital H
:
echo date( 'Y:m:d H:i:s', strtotime( $date1 )); // 12:10:00
echo date( 'Y:m:d H:i:s', strtotime( $date2 )); // 00:10:00
FWIW: Your time variables should be strings, with quotes.:
$date1 = "12:10:00";
$date2 = "00:10:00";