1. Not bearing a date
   "a dateless letter";
   - undated
    
2. Of such great duration as to preclude the possibility of being assigned a date
   "dateless customs"
    
3. Having no known beginning and presumably no end
   "the dateless rise and fall of the tides";
   - endless, sempiternal
    
4. Unaffected by time
   "few characters are so dateless as Hamlet";
   - timeless

See also: infinite, unaltered, unchanged, undatable