Definitions:

1. having no known beginning and presumably no end adjective
   • the dateless rise and fall of the tides
2. not bearing a date adjective
   • a dateless letter
3. of such great duration as to preclude the possibility of being assigned a date adjective
   • dateless customs
4. unaffected by time adjective
   • few characters are so dateless as Hamlet

Synonyms:

• endless
• sempiternal
• timeless
• undated