to update the date you need to write a filter. This can be done either on the acf/save_post filter like you are trying or https://www.advancedcustomfields.com/resources/acf-pre_save_post/
I would stick with what you’re trying. I really think that the date is not being updated because of a badly formatted date, see my last comment.
error checking can only be done in an ACF form by creating acf/validate_value filters https://www.advancedcustomfields.com/resources/acf-validate_value/
