Throughout books there are system date fields on Purchase Orders, Sales Orders, etc...

You can run validation on these BUT ONLY based on the date entered... example: it must be X days after or before Y.

However you can't validate the field is a user leaves it empty and you can't make the fields mandatory either which leaves you in a tight spot!

Add yet another custom field and make that field mandatory and run validation on that and then push the data to the system defined field.