What is Date Certaine?

Legal Definition
In French law. A deed is said to have a date certaine (fixed date) when it has been subjected to the formality of registration; after this formality has been complied with, the parties to the deed cannot by mutual consent change the date thereof. Arg. Fr. Merc. Law, 555.
-- Black's Law Dictionary
Legal Definition
(French) The date of recording an instrument.
-- Ballentine's Law Dictionary