The calendar used in the West is the Christian (Gregorian) calendar that is based on the solar movements, it started with the birth of Jesus Christ. On the other hand the Islamic (Hijri) calendar uses lunar movements and started with the emigration (Hijrah) of Prophet Muhammad(saw) from Mecca to Medina, which occurred on July 16th, 622.
After a lot of research and calculations a new calendar was worked out. Under the direct supervision of Huzoor the new Hijri-Shamsi calendar was adopted in 1940, which is indeed one of his brilliant achievements.
Although the calendar is based on solar calculations, the starting point in time is the migration of Prophet of Islam instead of Jesus Christ’s day of birth. According to this method 2000 AD translates into 1379 Hijri-Shamsi i.e. 1379 years have passed since the emigration of Prophet Muhammad, peace & blessings of Allah be on him, from Mecca to Medina.
Each month in the calendar is named after an important event that occurred in the history of Islam. The time frame in these months is the same as in the months of a Christian calendar. (Note: A lunar month is shorter by a couple of days than the solar month).