There's also the DateTime class which implements a function for comparison operators.
// $now = new DateTime();
$dtA = new DateTime('05/14/2010 3:00PM');
$dtB = new DateTime('05/14/2010 4:00PM');
if ( $dtA > $dtB ) {
echo 'dtA > dtB';
}
else {
echo 'dtA <= dtB';
}