I was trying to intercept the form submission before it was actually written to the DB using the hook/filter above. Is that not what this filter is for? I know it is doing something as the datetime is no longer written into the DB, it is just blank. Do I then need to update (SQL) that field? Seems like there should be a way to simply update the $_POST['acf']['seminar_date'] before it is inserted into the DB.
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