Note 409 — Chapter XXVII · THE LATER YEARS OF OMAR’S REIGN—DOMESTIC EVENTS. A.H. XVII.-XXIII. A.D. 638–644.
Reading edition
The calculation was already by strictly lunar notation of months, according to the Arab calendar; for that had been fixed by a Divine ordinance at the Farewell Pilgrimage. (Life of Mahomet, p. 486.) But the commencement of the era, and numbering of the years, was introduced only now. Note that the i is short in Hegira.