The word calendar is derived from Latin kalendae means first day of the month. It stands for dating system in the world. From ancient time man used time reckoning system based upon the movement of the sun and the moon. The altering phrases of moon provide good ground for a month. The 12 lunations completed a year of about 354 days. This is called lunar year. The earth completes a revolution in its orbit around the sun in 365.25 days. The period is called solar year and is divided into 12 months. The solar year is of fixed nature and computable with climate seasons around the world. The difference between solar and lunar year is about 11 days. Subsequently lunar year slides back 11 days per year and completes a round of passing through climatic season in 33 years.

Judaism, Christianity and Islam were prescribed with lunar calendar. The Christian calendar later was changed to solar. The present Christian calendar is based on Roman calendar, which was lunar in the beginning, covering 10 months of 304 days starting from March. Later, Numa (716-673 B.C.), the emperor of Rome added 2 months of January and February with a year comprising 355 days. By 46 B.C., the Roman Caesar started a new calendar, known as the Julian Calendar.

The Islamic calendar based on lunar cycle started from Friday, the Muharram 1, A.H., corresponding to July 16, 622 of the Julian Calendar. The Prophet's departure from Mecca is known as the hijra, the Arabic word for Migration. The era of the hijra is deemed, however, to have begun not on the precise day when he left Mecca, but on the first day of the Islamic year in which he reached in Kuba. This occurred almost two months earlier, on July 16th. Thus, 16 July 622 A.D. (A.D. = Anno Domini, the year of the Lord) coincides with 1 Muharram 1 A.H. (A.H. = Anno Hijrae, the year of the Hijra).

If one has a date in the Christian Calendar and intends to know the rough equivalent in the Islamic or Hijri Calendar, he must simply subtract 622. His answer will be only a rought approximation because the Muslim year is eleven days shorter than a solar year and there are consequently 33 Muslim years to every 32 years in the Christian calendar. To convert dates more accurately, it is therefore necessary to use the following formulae:

AH = AD - 622 + AD - 622
AD = AH + 622 - H