The Gregorian Calendar

What the Gregorian calendar is

The Gregorian calendar is the civil calendar used by most of the world. It divides the year into 12 months with different lengths and follows a leap-year pattern so the average calendar year stays close to the solar year. A common year has 365 days; a leap year has 366 days. The rule for leap years is standardised in the Gregorian system (divisible by 4, except century years, except century years divisible by 400); for details see Leap years.

The Gregorian calendar replaced the older Julian calendar because the Julian average year of 365.25 days ran slightly ahead of the solar year. That small difference accumulated over centuries and shifted the timing of the equinoxes.

The Julian calendar and its drift

The Julian calendar, introduced in 45 BC, added a leap day every fourth year. That gave an average year of 365.25 days. The actual solar year is about 365.2422 days, so the Julian calendar gained roughly 11 minutes per year. Over many centuries this drift moved the spring equinox earlier in the calendar year.

By 1582 the spring equinox fell about 10 days earlier in the calendar than it had at the Council of Nicaea in 325, the reference point for calculating the date of Easter. The Gregorian reform was designed to move it back and keep it there.

The 1582 reform

Pope Gregory XIII issued the papal bull Inter gravissimas in 1582 to reform the calendar. In countries that adopted the reform immediately, Thursday 4 October 1582 was followed directly by Friday 15 October 1582, skipping 10 calendar days. This removed the accumulated drift and adjusted the leap-year rule to keep the calendar aligned with the solar year more accurately.

Only the day numbers jumped. The cycle of weekdays was not interrupted: Thursday was followed by Friday as usual. Converting a date between the two calendars therefore never changes its weekday, only its day, month and sometimes year.

Adoption over time

Adoption of the Gregorian calendar was staggered by country and region. Catholic countries (such as Spain, Portugal and parts of Italy) adopted it in 1582. Many Protestant countries adopted later. Britain and its colonies switched in 1752: Wednesday 2 September 1752 was followed by Thursday 14 September 1752 (skipping 11 days by then, because the Julian drift had grown further). Russia adopted the Gregorian calendar in 1918, and Greece adopted it in 1923.

Old Style and New Style

Because of staggered adoption, the same historical event can appear with different dates depending on whether the record used the Julian calendar (Old Style) or Gregorian calendar (New Style). Before comparing dates from different sources, check which calendar was in force in that place at that time.

Proleptic Gregorian calendar and DateUtils

A “proleptic” calendar extends the rules backwards in time. DateUtils uses the proleptic Gregorian calendar for dates from year 1 to year 9999. In civil dating there is no year 0 (1 BC is followed by AD 1), and DateUtils starts at year 1. Because of this, dates before a country’s actual switch to Gregorian may differ from the dates recorded at the time. The DateUtils Day of week calculator follows the same proleptic rules when determining weekdays.

The rules every DateUtils calculator follows are listed in the methodology.

The 400-year cycle and weekdays

The Gregorian calendar’s leap-year cycle repeats every 400 years. There are exactly 146,097 days in 400 Gregorian years, which equals exactly 20,871 weeks. That means the pattern of dates and weekdays repeats every 400 years.

For example, 1 January 2000 was a Saturday, so 1 January 2400 will be one too. The DateUtils Date difference and Day of week tools apply these rules consistently across the supported range.

Practical consequences

• Historical dates can have two forms. George Washington was born on 11 February 1731 by the Julian calendar then used in the British colonies, which is 22 February 1732 in the Gregorian calendar. The year changes too, because England began the legal year on 25 March until the same 1752 reform moved it to 1 January.
• Old conventions survive in modern dates. The UK personal tax year starts on 6 April. It originally began on 25 March; the 11 days dropped in 1752 moved it to 5 April, and a later adjustment for the Julian leap day in 1800 moved it to 6 April.
• Weekday calculations for old dates are proleptic. If a document from before a country's switch gives a Julian date, entering that date into a Gregorian calculator gives the weekday of a different day. Convert the Julian date to its Gregorian equivalent first (add 10 days to Julian dates up to 28 February 1700, and 11 days from 29 February 1700 to 28 February 1800), then calculate.
• Genealogy records often mix both styles. Dates between 1 January and 24 March before 1752 are sometimes written with both years, such as 11 February 1731/32.