The Gregorian calendar is the calendar used by most societies today1. It is named after Pope Gregory XIII, who introduced the calendar in 1582.
The Gregorian calendar starts from 1 AD, the year that Jesus Christ was born, although no firm historical evidence has been uncovered to date the birth of Christ with certainty. The AD suffix stands for the Latin phrase anno Domini, meaning "in the year of our Lord". The AD suffix is sometimes replaced with CE (common era), although the numbering of the years does not change if the suffix is changed.
Years prior to the birth of Christ take the suffix BC (before Christ) or BCE (before common era), and are numbered backwards from 1 BC.
The Gregorian calendar replaced the Julian calendar used by Christian societies in 1582 AD. The Julian calendar mandated that a year was exactly 365.25 days long, rather than the more accurate 365.2425 days long. The slight inaccuracy in the Julian calendar caused the calendar to become increasingly out of synch with the seasons. To correct the inaccuracy, Pope Gregory deleted 10 days from 1582, and changed the method that was used to calculate leap years.
A leap year is a year that has one extra day added to February, making the year 366 days long, rather than the usual 365. The Julian calendar mandated that a leap year should fall on every year that is divisible by 4. The Gregorian calendar kept this rule, and further specified that a leap year should not occur in years that are divisible by 100, unless the year is also divisible by 400. For example, the year 1900 was not a leap year, but the year 2000 was a leap year. This slight modification allows the Gregorian calendar to stay aligned with the seasons.