The Collaborative International Dictionary of English v.0.48:

backdate \back"date\ 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]

   Note: Backdate is sometimes used incorrectly for postdate.

WordNet (r) 3.0 (2006):

backdate
    v 1: make effective from an earlier date; "The increase in tax
         was backdated to January"