The Collaborative International Dictionary of English v.0.48:

Timeless \Time"less\ (t[imac]m"l[e^]s), a.
   1. Done at an improper time; unseasonable; untimely. [R.]
      [1913 Webster]

            Nor fits it to prolong the heavenly feast
            Timeless, indecent.                   --Pope.
      [1913 Webster]

   2. Done or occurring before the proper time; premature;
      immature; as, a timeless grave. [Obs.]
      [1913 Webster]

            Must I behold thy timeless, cruel death? --Shak.
      [1913 Webster]

   3. Having no end; interminable; unending. "Timeless night and
      chaos." --Young.
      [1913 Webster]

WordNet (r) 3.0 (2006):

timeless
    adj 1: 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:

95 Moby Thesaurus words for "timeless":
   abiding, age-old, ageless, all-knowing, all-powerful, all-seeing,
   all-wise, almighty, ancient, antique, auld, boundless, ceaseless,
   changeless, coeternal, constant, continual, continuous, creating,
   creative, dateless, deathless, elderly, endless, eternal,
   eternally the same, eterne, ever-being, ever-durable, ever-during,
   everlasting, everliving, glorious, good, hallowed, highest, hoary,
   holy, immemorial, immortal, immutable, incessant, indestructible,
   infinite, interminable, just, limitless, loving, luminous,
   majestic, making, merciful, never-ceasing, never-ending, nonstop,
   nonterminating, nonterminous, numinous, of old, of yore, olamic,
   old, old as Methuselah, old as history, old as time, old-time,
   olden, omnipotent, omnipresent, omniscient, one, perdurable,
   permanent, perpetual, radiant, sacred, sempiternal, shaping,
   sovereign, steady, supreme, ubiquitous, unbounded, unceasing,
   unchanged, unchanging, undefined, undying, unending,
   unintermitting, uninterrupted, unlimited, unremitting, venerable,
   without end