reckoner

noun3

1. •
   One who reckons.

   • “No. Not quite a month.—It is only four weeks to-morrow since I left Mansfield.” “You are a most accurate and honest reckoner. I should call that a month.” — 1814
2. •
   An ordinator.
3. •
   (no longer used) An accountant; one who computes or calculates.