In order for that to work $time has to be a timestamp. You cannot pass in "10:00" or something like $time = date('H:i', '10:00'); which is what you seem to do, because then I get 0:30 and 1:30 as results too.
Try
$time = strtotime('10:00');
As an alternative, consider using DateTime (the below requires PHP 5.3 though):
$dt = DateTime::createFromFormat('H:i', '10:00'); // create today 10 o'clock
$dt->sub(new DateInterval('PT30M')); // substract 30 minutes
echo $dt->format('H:i'); // echo modified time
$dt->add(new DateInterval('PT1H')); // add 1 hour
echo $dt->format('H:i'); // echo modified time
or procedural if you don't like OOP
$dateTime = date_create_from_format('H:i', '10:00');
date_sub($dateTime, date_interval_create_from_date_string('30 minutes'));
echo date_format($dateTime, 'H:i');
date_add($dateTime, date_interval_create_from_date_string('1 hour'));
echo date_format($dateTime, 'H:i');