septuagesimal

from The Collaborative International Dictionary of English v.0.48

Septuagesimal \Sep`tu*a*ges"i*mal\, a.
   Consisting of seventy days, years, etc.; reckoned by
   seventies.
   [1913 Webster]

         Our abridged and septuagesimal age.      --Sir T.
                                                  Browne.
   [1913 Webster]