Как проще всего сравнивать даты?

$date1 = ("2011-08-19, 21:33");
$date2 = ("2011-08-19, 16:40");
 
if ($date1 > $date2) {
     echo 'true';
} else echo 'false';

$str_date = "2001-01-01 15:20:20";
$date_elems = explode(" ",$str_date);
$date = explode("-", $date_elems[0]);
$time = explode(":", $date_elems[1]); 
$result =  mktime($time[0], $time[1],$time[2], $date[1],$date[2], $date[0]);

$date1 = ("2011-08-19, 21:33");

$str = '2010-12-12 12:11:10';// $str - строка
$timestamp = strtotime($str);// $timestamp - уже дата
if ( $timestamp === FALSE){
 echo "Строка ($str) недопустима";
}
else{
 echo date('Y-m-d H:i:s', $timestamp);
}

 $timestamp = '2010-12-31 22:11:10';

$str = '2010-12-12 23:11:10';
$timestamp = strtotime($str);
if ( $timestamp === FALSE){
 echo "Строка ($str) недопустима";
}
else{
 echo date('Y-m-d H:i:s', $timestamp);
 echo "<br />";
 $q= strtotime(" + 3 hours", $timestamp);
 echo date('Y-m-d H:i:s', $q);
}

strtotime(date('Y-m-d H:i:s GMT') . ' + 3 hours'));

date('d.m.Y', strtotime(date('Y-m-d H:i:s GMT') . ' + 3 hours')));

$minus = -3;
$plus = 3;
 
$year = 2011;
$month = 4;
$day = 16;
$hour = 2;
$minute = 0;
 
$date = $year . "-" . $month . "-" . $day . " " . $hour . ":".$minute . ":00";

$date = strtotime($killed_date);

$a = date('Y-m-d H:i:s', $date);

$start = date("Y-m-d H:i:s", strtotime($a . " + $minus hours"));

" + $minus hours" // это строка " + -3 hours", а надо " -3 hours"