Virtually all advanced cultures come up with one or more systems for keeping track of time.  The ancient Egyptians used at least two and perhaps as many as four systems concurrently.  For some fifteen hundred years, the west has used a system that originated during the early Middle Ages, a system known as the Christian Era Date.  (An era date is any fixed point in time from which a society may date events both foward and backward.)

By the early Middle Ages, Christianity had become the dominant religion throughout the Mediterranean world and was rapidly spreading through northern Europe.  Consequently, throughout this region, the birth of Jesus had come to be thought of as the most important event in human history.  This led several leading scholars of the period to begin using the year they calculated as having witnessed the birth of Jesus as a point from which to date all other events.  As western writers picked up the idea, the date traditionally associated with the birth (almost certainly the wrong year) became the Christian Era Date.  

In counting forward, we speak of such and such a year A.D., an abbreviation for Anno Domini or “in the year of our Lord.”  If, on the other hand, we count backward, we designate the year as being B.C., “Before Christ.”   Thus, Columbus reached the American continent in 1492 A.D. or 1492 years after the birth of Jesus, while Rome was founded in 753 B.C., 753 years before his birth.

Several societies other than the west have arrived at the extremely useful concept of an era date.  The most famous of these rival systems is called the Islamic Era Date, a system that has prevails in Moslem landed for just about as long as the Christian system has been in existence.  

Not surprisingly, Islam chose for its era date a critical year in the life of its prophet, Mohammed.  Mohammed was a wealthy merchant who lived in the Arabian city of Mecca in the seventh century.  In 611 A.D., at the age of forty, he had a religious vision, as a result of which he began to preach a new religion.  At first, the people of Mecca greeted his preaching with considerable skepticism, as people tend to do with prophets.  When he kept trying to convert them, thereby endangering the traditional polytheistic pilgrim trade that flocked to the city annually, the opposition turned ugly.  Mohammed, his family, and close followers were forced to flee from Mecca and take up residence in the neighboring city of Medina.  This flight, known as the Hejira, occurred in 622 A.D. and marked the critical turning point in Mohammed’s career.  Thereafter, his fortunes continued to rise until in 632, the year of his death, he was the master of Arabia and Arab armies were poised for an expansion that would in the next century take from the gates of Paris in the west to the Indus River in the east.  Consequently, 622 A.D. became the Islamic Era Date, i.e. the first year in the Islamic calendar, where dates are said to be either B.H. (Before the Hejira) or A.H. (After the Hejira).

To convert from one calendar to the other requires understanding the relationship of the two:

The first year in the Islamic Calendar = the 622nd year in the Christian calendar (622 A.D.)

Therefore, to find the Islamic date for the first year in the Christian calendar, one must count back 622 years, an operation that puts one at 622 B.H. (Before the Hegira.)  This would be the Islamic way of expressing the date for the birth of Jesus.

How would you find the year 1000 A.D. (the first millenium) in the Islamic calendar?

Step 1:  Count the number of years that elapsed between 622 and 1000.  The answer is 378.
Step 2:  Count forward on the Islamic calendar 378 years starting at what amounts to the zero point.
Answer:  1000 A.D. = 378 After the Hejira = 378 A.H.

There is one complication.  The procedure just outlined assumes a year of equal length in each system.  This is a mistaken assumption.  The Christian calendar is based on the solar year of 365 and 1/4 days; the Islamic calendar uses a lunar year of approximately 360 days.  As a result, over time, the two calendars diverge from one another.  While there exists a formula that makes it possible to calculate this divergence,  for purposes of the test, students may assume a year of equal length and use the above procedure to undertake a conversion.

One final word about dating:  Despite the existence in the past of many other systems for recording time, in the last few centuries, the cultural and political dominance exercised by the west over the rest of the world has led many societies to abandon their traditional systems and adopt instead the western calendar, centered on the birth of Jesus.  At the same time, a movement to do away with the religious overtones of this calendar has been gaining strength.  Many non-Christian societies adopting the western system of dating have come to refer to the western system not as the Christian Era, but as the Common Era.  Consequently, dates which would traditionally be B.C. (Before Christ) are being expressed as BCE or Before the Common Era.  Dates which were AD are increasingly spoken of simply as C.E., In the Common Era.  Many western scholars have followed the lead of non-western societies in this respect, referring to dates as BCE and CE.