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]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "postdated":
   ahead of time, anachronistic, antedated, beforehand, behind time,
   behindhand, dated, early, foredated, late, metachronistic,
   misdated, mistimed, out of date, out of season, overdue,
   parachronistic, past due, prochronistic, tardy, unpunctual,
   unseasonable