The Collaborative International Dictionary of English v.0.48:

Postdate \Post"date`\, n.
   A date put to a bill of exchange or other paper, later than
   that when it was actually made.
   [1913 Webster] Postdiluvial

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]

The Collaborative International Dictionary of English v.0.48:

Postdate \Post"date`\, a.
   Made or done after the date assigned.
   [1913 Webster]

         Of these [predictions] some were postdate; cunningly
         made after the thing came to pass.       --Fuller.
   [1913 Webster]

WordNet (r) 3.0 (2006):

postdate
    v 1: be later in time; "Tuesday always follows Monday" [syn:
         postdate, follow] [ant: antecede, antedate,
         forego, forgo, precede, predate]
    2: establish something as being later relative to something else
       [ant: antedate, foredate, predate]