For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
// build a date object from your date placeholder
$myDate = new DateTime('{yourtable___yourdate}');
// add your week placeholder
$myDate->modify('+{yourtable___yourweeks} week');
// return formatted date
return $myDate->format('Y-m-d');