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
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$date1 =new DateTime('{yourtable___field1_raw}');
$date2 =new DateTime('{yourtable___field2_raw}');
return $date2->diff($date1)->format("%a");
use a calc element