PHPタイムスタンプについて

$t = time();
$t1 = mktime(0,0,0,date("m",$t),date("d",$t),date("Y",$t));
$t2 = mktime(0,0,0,date("m",$t),1,date("Y",$t));
$t3 = mktime(0,0,0,date("m",$t)-1,1,date("Y",$t));
$t4 = mktime(0,0,0,1,1,date("Y",$t));
$e1 = mktime(23,59,59,date("m",$t),date("d",$t),date("Y",$t));
$e2 = mktime(23,59,59,date("m",$t),date("t"),date("Y",$t));
$e3 = mktime(23,59,59,date("m",$t)-1,date("t",$t3),date("Y",$t));
$e4 = mktime(23,59,59,12,31,date("Y",$t));
//  
echo date("   Y-m-d H:i:s",$t)." $t
";
echo date("     Y-m-d H:i:s",$t1)." $t1
";
echo date("     Y-m-d H:i:s",$t2)." $t2
";
echo date("     Y-m-d H:i:s",$t3)." $t3
";
echo date("     Y-m-d H:i:s",$t4)." $t4
";
echo date("     Y-m-d H:i:s",$e1)." $e1
";
echo date("     Y-m-d H:i:s",$e2)." $e2
";
echo date("     Y-m-d H:i:s",$e3)." $e3
";
echo date("     Y-m-d H:i:s",$e4)." $e4
";

結果:現在2011-05-24 15:42:55 1306222975今日起点2011-05-24 00:00:00 1306166400今月起点2011-05-01 00:00 1304179200先月起点2011-04-01 00:00:00 1301587200今年起点2011-01-01 00:00:00 1293811200今日終点2011-05-24 23:59 1306252799今月終点2011-05-31 23:59 1306857599先月終点2011-04-30 23:59 1304179199今年のゴール2011-12-31 23:59:59 132534799