See John 17:3.
At the bottom of page [23], Burgess wrote two numerical calculations. In one, which was canceled, Burgess attempted to calculate JS’s age: “1844 | [-] 1805 | [=] 39.” In the other, Burgess calculated the year in which JS would turn eighty-five: “1805 | [+] 85 | [=] 1890.” (Burgess, Journal and Notebook, [23].)
Burgess, James. Journal and Notebook, Oct. 1841–Dec. 1848. James Burgess, Journals, 1841–1848. CHL.