The Gregorian calendar

Introduced October 1582

Timekeeping — the calendar most of the world now uses

The civil calendar of most of the world, introduced by Pope Gregory XIII in the bull Inter gravissimas of 24 February 1582 to correct the drift of the Julian calendar. By dropping three leap days every four centuries — century years are leap years only if divisible by 400 — it shortened the average year to 365.2425 days, very close to the true solar year of about 365.2422. To restore the spring equinox to 21 March, which mattered for calculating Easter, ten days were deleted outright: Thursday 4 October 1582 was followed by Friday 15 October.

How to spot them

▸Century years are leap years only when divisible by 400 — 1900 was not, 2000 was
▸Ten days vanished in October 1582; eleven vanished in Britain in September 1752
▸Introduced to stop Easter drifting away from the spring equinox
▸Old Style and New Style dates, and Russia's 'October' Revolution falling in November

Key facts

Introduced by the papal bull Inter gravissimas, dated 24 February 1582 (1582)

Issued by Pope Gregory XIII, after whom it is named

Thursday 4 October 1582 was followed by Friday 15 October (1582)

Ten days deleted to put the equinox back on 21 March; the cycle of weekdays was not interrupted

Century years are leap years only when divisible by 400

So 1600 and 2000 were leap years, while 1700, 1800 and 1900 were not

An average year of 365.2425 days, from a scheme by the Calabrian doctor Aloysius Lilius

Christopher Clavius argued the case in an 800-page volume and insisted on one sudden correction

Britain and its colonies skipped eleven days in September 1752 (1752)

The same Act moved the start of the legal year from 25 March to 1 January

Greece was the last country in Europe to adopt it, in 1923 (1923)

Russia had switched in 1918, which is why the October Revolution is marked in November

Sweden bungled a gradual changeover so badly that it once had a 30 February (1712)

It reverted to the Julian calendar by adding a second leap day that year

Key ideas

The Julian year of 365.25 days is about 11 minutes too long, so it slips roughly one day every 128 years. By 1582 that had accumulated to ten days since the Council of Nicaea in 325.
The 400-year cycle contains exactly 146,097 days, which is 20,871 whole weeks — so the calendar repeats exactly every four centuries.

Quick-fire facts

•The reform was announced in the bull Inter gravissimas of 24 February 1582 by Pope Gregory XIII
•The scheme was devised by Aloysius Lilius (Luigi Lilio) and defended in detail by the Jesuit mathematician Christopher Clavius
•The mean Gregorian year is 365.2425 days against a tropical year of about 365.2422 — an error of roughly one day in 3,000 years
•The Julian calendar's error is about one day in 128 years
•The motive was ecclesiastical: to fix the date of Easter, which depends on the spring equinox falling on 21 March
•Catholic Spain, Portugal, Poland–Lithuania and the Papal States changed on the date Gregory set; France followed in December 1582
•Britain and its empire adopted it in September 1752, deleting eleven days; the story of 'Give us our eleven days' riots is almost certainly a myth
•Russia changed in 1918, Greece — the last European country, for civil purposes — in 1923
•Many Orthodox churches still use the Julian calendar for religious feasts, which is why Orthodox Christmas falls on 7 January
•Sweden's attempt at a gradual transition produced the unique date 30 February 1712

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Sources

Edward M. Reingold and Nachum Dershowitz — Calendrical Calculations
Pope Gregory XIII — Inter gravissimas The papal bull of 24 February 1582 that promulgated the reform