Is it possible to add a date validation to a descriptive flexfield?

Summary:

Per sponsors' requirements, UCD needs to add two flexfields to the Invoice header for control of the billing period. The information that would be entered is the start and end date of the billing period. All the available flexfields for Invoice Header are descriptive. We would like to add a date validation to ensure that the information that is entered corresponds to a date. Also, would like to make sure that the start date is before or equal to the end date. Those two fields would be included in the invoice format. UCD programmed