Deactivate ValidTimeStateFieldType

I'm working with UTCDateTime fields, but I'm having some troubles because of the index, the table is DocuRef, i make an index with "RecId", "ValidFrom", "ValidTo", there is no data in the columns "ValidFrom" and "ValidTo" but i cannot go back the property ValidTimeStateFieldType to "none", the columns "ValidFrom" and "ValidTo" are empty, and I'm trying to go back because when i try to add some field in the VendTable from in the part of attachment, appears a message of error and the program terminate. Any solution for some of my problems?

Thanks for your attention.