Form Submission Validate One Field Against Another?

I wonder if anyone can assist me here. I am trying to find the best way to validate a field against another field. In this case I need to make sure when the user enters the date of birth that the date is not prior to another date in the database (admission date). I would like to do the validation on submit before the other data from the field is submitted to the database.

I have seen a lot of forms validation that can compare the inputted value to a range of values, but I need to say "is the birth date prior to the admission date? If NO then give a message to the user to recheck the date". Sounds easy, but I haven't found a clean way to do it.