Napr.:

$text = "24.12.2008 12:30:15";
@list($date, $time) = explode(" ", $text);
@list($day, $month, $year) = explode(".", $date, 3);
@list($hour, $minute, $second) = explode(":", $time, 3);
$timestamp = mktime((int)$hour, (int)$minute, (int)$second, (int)$month ? (int)$month : 1, (int)$day ? (int)$day : 1, (int)$year);

$text = date("d.m.Y H:i:s", $timestamp);