Hegira, HEJRA, or HIJRA

Hegira, HEJRA, or HIJRA (an Arab word which means 'going away'), the term commonly used to indicate Mohammed's flight from Mecca, 13th September 622 A.D. In 639 or 640 the Calif Omar instituted a new Moslem calendar, to begin with the first day of the first month of the year in which the flight took place. The Mohammedan year, as a lunar year, is shorter than ours by 10 days, 21 hours, and 14$\backslash frac\{2\}\{3\}$ seconds. A rough and ready method for finding the year in our calendar corresponding to a given year in the Mohammedan is to subtract from the latter $\backslash frac\{1\}\{33\}$ of itself and add 622 to the remainder. To find the precise year and day, multiply the year of the Hegira by 970224, strike off from the product six decimal figures, and add 621.5774; this will give the year of the Christian era; and the day of the year is got by multiplying the decimal figures by 365.