v. t. 1. to make effective from an earlier date; to make retroactive. The increase in tax was backdated to January. Syn. -- back-date [ WordNet 1.5 ]
2. to affix a date earlier than the present date; -- sometimes done for fraudulent purposes. Opposite of postdate. Syn. -- back-date; predate; antedate. [ PJC ]
Backdate is sometimes used incorrectly for postdate.