The Gregorian calendar is the solar calendar introduced by Pope Gregory XIII in 1582. It is the international civil standard for dates today.
Mean year and leap rules
The Gregorian calendar uses a mean year of 365.2425 days. Leap years follow 3 rules confirmed by the United States Naval Observatory:
- Years divisible by 4 are leap years.
- Century years are not leap years, unless...
- They are also divisible by 400.
So 2000 was a leap year, 2100 will not be, and 2400 will be. This pattern keeps the calendar aligned with the tropical year to within 1 day per 3,000 years.
Replacement of the Julian calendar
The Gregorian calendar replaced the Julian calendar, which used a simpler leap-year rule of every 4 years without exception. By 1582, the Julian system had drifted 10 days behind the solar year. Pope Gregory XIII's reform skipped 4 to 14 October 1582 in Catholic countries to restore alignment. Britain and its colonies adopted the change in 1752, skipping 11 days.
Basis for ISO 8601
ISO 8601 is built on the Gregorian calendar. Every ISO date format, including week dates and ordinal dates, uses Gregorian year, month, and day values.