Quite strange behavior :
perhaps test this :
$today = time();
$datum=date(YmdHis,$today);
Normally may bug the same way...
So it is perhaps because your local time and server time isn't the same.
Try to find the constance of time difference between server and clients....
use the php funtion "mktime" to generate your date
or this code :
$hourd = "8"; // hours difference between server time and local time
$timeadjust = ($hourd * 3600);
$output = date("l, d F Y h:i a",time() + $timeadjust);