You can get unix timestamp from Date object as follows (see Date.prototype.getTime)
var timestamp = '@' + Math.round(datumUhrzeit.getTime()/1000);
Then when sent on server simply create new datetime object
$datumUhrzeit = new DateTime($timestamp);
If you can't use javascript to create timestamp and you get the the data from form directly you can do something like this, remember to set the timezone:
$datum = $_GET['datum'];
$uhrzeit = $_GET['uhrzeit'];
$datumUhrzeit = DateTime::createFromFormat('Y-m-d H:i:s', $datum . ' ' . $uhrzeit, new DateTimeZone('Europe/Berlin'));
Now as you have saved your date to the database and retrieved it, you can send it back
print $datumUhrzeit->format('U'); // This will print the time as unix timestamp
After that you would create your javascript date object with just the timestamp
var datumUhrzeit = new Date(timestamp * 1000); // timestamp from above
If you for some reason don't want to use unix timestamp you can print it in desired format with format method. Remember to set the timezone beforehand
$datumUhrzeit->setTimezone(new DateTimeZone('Europe/Berlin'));
print $datumUhrzeit->format('Y-m-d H:i:s');
Because javascript doesn't work well with timezones I would advocate you to use unix timestamps when you can. This way you have less problems with timezones.