The Collaborative International Dictionary of English v.0.48:

datable \dat"a*ble\, a.
   That may be dated; having a known or ascertainable date.
   "Datable almost to a year." --The Century.

   Syn: dateable. [1913 Webster]

WordNet (r) 3.0 (2006):

datable
    adj 1: that can be given a date; "a concrete and datable
           happening"- C.W.Shumaker [syn: datable, dateable]
           [ant: undatable]