From BiblePortal Wikipedia

Webster's Dictionary [1]

(n.) The flight of Mohammed from Mecca, September 13, A. D. 622 (subsequently established as the first year of the Moslem era); hence, any flight or exodus regarded as like that of Mohammed.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]

an Arabic word signifying flight (Hejra), now used to designate the epoch from which the Mohammedans compute time. The flight of Mohammed from Mecca to Medina is fixed by the Mohammedans on July 15, A.D. 622. The process of converting the years of the Hegira into the date after the birth of Christ is as follows. Divide the given number by thirty (the quotient expresses the intercalary cycles elapsed since the Hegira, the remainder represents the number of years elapsed in the current intercalary cycle); multiply the quotient by 10,631 (the number of days contained in an intercalary cycle), adding to the product the sum of the days contained in the elapsed years of the current cycle, the days of the elapsed current months of the current year up to the time of reckoning, and to the result add again 227,015 (the number of days elapsed between Jan. 1 of the year 1, and July 15, 622, the date of the Hegira). The stun of days thus obtained is most readily converted into Julian years by dividing it by 1461 (the number of days in a Julian intercalary period), then multiplying the quotient by four, and adding to the product the number of whole years contained in the remainder of the division, which is obtained by dividing this remainder by 365. The number of days still remaining shows the day of the month in the current Julian year. Or else the following proportion may be made use of (T representing any date in the Turkish. calendar, and C the corresponding date in the Julian calendar): C =0.970203 T + 621.567785, and T = 1.030712 C 64.65745. If the date is subsequent to the Gregorian reform in the calendar, which can only be the case for modern times, then the Turkish date must first be converted into the Julian, which is then altered to the Gregorian by adding ten days to it for the period extending from Oct. 5, 1582, to the end of February, 1700; eleven days after the latter until the end of February, 1800, and twelve days for all subsequent dates. In making this reduction, the difference between the time at which the day begins in the Turkish and in the Christian calendar must be taken into consideration whenever the time of day of the event calculated is known, as it may make a difference in the date of one day more or less. The Turkish year begins at the end of July. The year 1859 A.C. is in their calendar 1275-76. A simpler mode of reduction, but not strictly accurate, is as follows: The Mohammedan year a lunar year of 354 days, and therefore 33 Mohammedan years =32 Christian. To reduce years of the Hegira, therefore, to years of the Christian era, subtract one from every thirty-three years, and add 622. Thus A.D. 1861 = 1277 of the Hegira. Pierer, Universal Lexikon, 8:721.