I haven't found a reference to that date that's earlier than 4.4BSD's /usr/share/calendar, which includes 09/20 Harlan Herrick runs first FORTRAN program,

I haven't found a reference to that date that's earlier than 4.4BSD's /usr/share/calendar, which includes 09/20 Harlan Herrick runs first FORTRAN program, 1954 In The History of FORTRAN I, II, III (from HOPL I and available at your CHM link), Backus writes: The FORTRAN compiler (or “translator” as we called it then) was begun in early 1955, although a lot of work on various schemes which would be used in it had been done in 1954; e.g. Herrick had done a lot of trial programming to test out our language [....] So that's consistent.