Project descriptive flex field validation to Date DFF in Project Header screen

Summary:

In project's header screen , we have created a project descriptive flex field for date field. user can select the date from this field but now we got a ask that we need to put a validation like the entered cannot be future date, how to set this validation

i have followed below steps

1. created a Date DFF by using Manage Project descriptive flex field and assigned Value set to this DFF
2. attaching this value set details and code used , please find screenshot.
3. im able to get sysdate in DFF field, but when i select the value , screen getting refreshed but not populating the Value in the field. attaching that screenshot too.