2009-04-01 08:40:00
↑の時間の30分後を取得する方法。

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$date= '2009-04-01 08:40:00';
preg_match("/(\d{4})\D(\d\d?)\D(\d\d?)\s+(\d\d?):(\d\d?):(\d\d?)/s", $date, $d);
$d= localtime(60*30+mktime($d[4], $d[5], $d[6], $d[2], $d[3], $d[1]));
printf("%d-%02d-%02d %02d:%02d:%02d", $d[5]+1900,$d[4]+1,$d[3],$d[2],$d[1],$d[0]);