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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Sounds like a good idea, indeed. Just don't use date and jdate simultaneously.Would it be a good idea to use jdate for new projects, but current ones are ok with the old element for "some" time?
Don't expect much "old" to function with J! 4 - if anything at all. Plus, allow plenty of time still before you'll be able to do with J! 4 what you can do with J! 3 today.And with Joomla 4 the old date element would still function?
The new jdate doesn't have (at least until now) the "Allow date" functionality (= restrict selectable dates in the calendar widget). So as always: it depends...Would it be a good idea to use jdate for new projects