It seems you are using Formidable for the registration form. ACF saves the data in this format: YYYYMMDD, so you need to save the extra data from the form to the database in that format. If Formidable saves the date in this format: YYYY-MM-DD, you need to change it to this format: YYYYMMDD. Please ask Formidable support if they have a hook to do it.
I hope this makes sense.
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