The Collaborative International Dictionary of English v.0.48:

dateless \date"less\, a.
   1. Without date; having no fixed time.
      [1913 Webster]

   2. not having a social companion for an occasion; as, to be
      dateless for the prom.
      [PJC]

WordNet (r) 3.0 (2006):

dateless
    adj 1: having no known beginning and presumably no end; "the
           dateless rise and fall of the tides"; "time is endless";
           "sempiternal truth" [syn: dateless, endless,
           sempiternal]
    2: of such great duration as to preclude the possibility of
       being assigned a date; "dateless customs"
    3: not bearing a date; "a dateless letter" [syn: dateless,
       undated]
    4: unaffected by time; "few characters are so dateless as
       Hamlet"; "Helen's timeless beauty" [syn: dateless,
       timeless]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "dateless":
   age-old, ageless, ancient, antique, auld, ceaseless, coeternal,
   constant, continual, continuous, elderly, endless, eternal, eterne,
   ever-being, ever-durable, ever-during, everlasting, everliving,
   hoary, immemorial, incessant, indestructible, infinite,
   interminable, never-ceasing, never-ending, nonstop, nonterminating,
   nonterminous, of old, of yore, olamic, old, old as Methuselah,
   old as history, old as time, old-time, olden, perdurable,
   permanent, perpetual, sempiternal, steady, timeless, unceasing,
   unending, unintermitting, uninterrupted, unremitting, venerable,
   without end