Unfortunately, the JS documentation has always been a little light here. It’s up to us to figure it out and help others if we’re inclined to do so.
I did manage to find and old thread dealing with auto filling an end date based on the start date. The information is for the old JS, but it does show the basics of dealing with date fields. Some of it should still be valid https://support.advancedcustomfields.com/forums/topic/autocomplete-end-date-when-start-date-is-selected-in-jquery-date-picker/
