Definition of "reckoner"

“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 July, [Jane Austen], chapter XLII, in Mansfield Park: […], volume I, London: […] T[homas] Egerton, […]

middle-aged (-ajd), between youth and old age, variously reckoned to suit the reckoner

1959 , “middle-aged”, in William Geddie, editor, Chambers Twentieth Century Dictionary, revised edition, page 672

Some experienced New York political reckoners think the real race, in the end, will be between Democrat Richard Ottinger and Mr. Buckley.

1970 July 25, Anthony Lewis, “The Charm of Mr. Buckley”, in The New York Times‎