One thing to keep in mind with a form rule on a date field is if the date field is the last or only thing they interact with on the record they usually need to click out of the field to make the form rule fire. So if they enter a wrong date and then immediately go down and save, the record will save with the invalid date.
Another issue I have encountered using form rules for date validation is there are other steps a user can take to input an invalid date even without that being the last field they click. In the past I've tried to make it so it clears the date when it is invalid but that fails randomly. I haven't been able to 100% pin it down but if they interact with the date field and input an invalid one and the form rule kicks in sometimes it will not kick in a second time and allow the user to input whatever they want without another error and they can save it.
Form rules are not perfect for data validation so it might come down to training and just making sure the user fully understands what they should be doing. It can help to add helper text near the field that shows what you expect to be entered and what a bad entry would be, if this field is a problem.
