The Collaborative International Dictionary of English v.0.48:

Postdate \Post"date`\, v. t. [imp. & p. p. Postdated; p. pr. &
   vb. n. Postdating.] [Pref. post- + date.]
   1. To date after the real time; as, to postdate a contract,
      that is, to date it later than the time when it was in
      fact made.
      [1913 Webster]

   2. To affix a date to after the event.
      [1913 Webster]