Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. To affix a date to after the event.