The Christian calendar, also known as the Western calendar or the Gregorian calendar, is the calendar currently used in most of the world. It is based on the solar calendar, with a year of 365 days divided into 12 months, with an extra day added to February in leap years, which occur every four years.
The Christian calendar is considered to start from the birth of Jesus Christ, or the Incarnation of Christ, and it is also called "Anno Domini" (A.D.) in Latin, which means "In the year of our Lord". The years before Jesus Christ were designated "Before Christ" (B.C.).
It was first introduced by Pope Gregory XIII in 1582 as a reform of the Julian calendar, which had been in use for over 1,500 years but had accumulated errors. The reform aimed to align the calendar more closely with the solar year, and it is known as the "Gregorian calendar" after Pope Gregory XIII. The Christian calendar is widely used and accepted worldwide in different cultures and religions as a standard of civil time.