Gregorian calendar

For the calendar of religious holidays and periods, see Liturgical year. For this year's Gregorian calendar, see Leap year starting on Monday.

2024 in various calendars

Gregorian calendar	2024 MMXXIV
Ab urbe condita	2777
Armenian calendar	1473 ԹՎ ՌՆՀԳ
Assyrian calendar	6774
Baháʼí calendar	180–181
Balinese saka calendar	1945–1946
Bengali calendar	1431
Berber calendar	2974
British Regnal year	2 Cha. 3 – 3 Cha. 3
Buddhist calendar	2568
Burmese calendar	1386
Byzantine calendar	7532–7533
Chinese calendar	癸卯年 (Water Rabbit) 4721 or 4514     — to — 甲辰年 (Wood Dragon) 4722 or 4515
Coptic calendar	1740–1741
Discordian calendar	3190
Ethiopian calendar	2016–2017
Hebrew calendar	5784–5785
Hindu calendars
- Vikram Samvat	2080–2081
- Shaka Samvat	1945–1946
- Kali Yuga	5124–5125
Holocene calendar	12024
Igbo calendar	1024–1025
Iranian calendar	1402–1403
Islamic calendar	1445–1446
Japanese calendar	Reiwa 6 (令和６年)
Javanese calendar	1957–1958
Juche calendar	113
Julian calendar	Gregorian minus 13 days
Korean calendar	4357
Minguo calendar	ROC 113 民國113年
Nanakshahi calendar	556
Thai solar calendar	2567
Tibetan calendar	阴水兔年 (female Water-Rabbit) 2150 or 1769 or 997     — to — 阳木龙年 (male Wood-Dragon) 2151 or 1770 or 998
Unix time	1704067200 – 1735689599

The Gregorian calendar, also called the Western calendar and the Christian calendar, is internationally the most widely accepted and used civil calendar. It has been the unofficial global standard for decades, recognized by international institutions such as the United Nations and the Universal Postal Union.

The calendar was a reform in 1582 to the Julian calendar. The motivation for the reform was to bring the date for the celebration of Easter to the time of the year in which the First Council of Nicaea had agreed upon in 325. Because the spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered this steady drift in the date of Easter undesirable. The reform was adopted initially by the Catholic countries of Europe. Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar and adopted the Gregorian reform after a time, for the sake of convenience in international trade. The last European country to adopt the reform was Greece as late as 1923.

The Gregorian reform contained two parts: a reform of the Julian calendar as used prior to Pope Gregory XIII's time and a reform of the lunar cycle used by the Church, with the Julian calendar, to calculate the date of Easter. The reform was a modification of a proposal made by Aloysius Lilius. His proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years. Lilius also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.

The Gregorian reform modified the Julian calendar's scheme of leap years as follows:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.

In addition to the change in the mean length of the calendar year from 365.25 days (365 days 6 hours) to 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year, the Gregorian calendar reform also dealt with the accumulated difference between these lengths. Between AD 325 (when the First Council of Nicaea was held, and the vernal equinox occurred approximately 21 March), and the time of Pope Gregory's bull in 1582, the vernal equinox had moved backward in the calendar, until it was occurring on about 11 March, 10 days earlier. The Gregorian calendar therefore began by skipping 10 calendar days, to restore 21 March as the date of the vernal equinox.

Though not part of the Gregorian calendar itself, the reformed calendar continued the previous year-numbering system (Anno Domini), which counts years from the traditional date of the nativity, originally calculated in the 6th century and in use in much of Europe by the High Middle Ages. This year-numbering system, now also called the Common Era, is the predominant international standard today.

Description

The Gregorian calendar is a solar calendar. A regular Gregorian year consists of 365 days and in a leap year, an intercalary or leap day is added as 29 February making the year 366 days. Normally a leap year occurs every 4 years, but the Gregorian calendar omits 3 leap days every 400 years, unlike the Julian calendar, which retains those leap days.

A Gregorian year is divided into twelve months, as follows:

No.	Name	Days
1	January	31
2	February	28 or 29
3	March	31
4	April	30
5	May	31
6	June	30
7	July	31
8	August	31
9	September	30
10	October	31
11	November	30
12	December	31

Although the month length pattern is irregular, for all but February, it can be calculated with the arithmetic expression L = 30 + { [ M + floor(^M/₈) ] MOD 2 }, where L is the month length in days and M is the month number 1 to 12. For February (M = 2), subtract one from the result during leap years, or subtract 2 from the result during common years.

Gregorian years are identified by consecutive year numbers. The cycles repeat completely every 146,097 days, which equals 400 years, and which also happens to be 20,871 seven-day weeks.

Of these 400 years, 303 are regular years of 365 days and 97 are leap years of 366 days. A calendar mean year is 365+⁹⁷/₄₀₀ days = 365.2425 days = 365 days, 5 hours, 49 minutes and 12 seconds. The same result is obtained by summing the fractional parts implied by the rule: 365 + 1⁄4 − 1⁄100 + 1⁄400 = 365 + 0.25 − 0.01 + 0.0025 = 365.2425

For a better representation, consider Zeller's Congruence.

A calendar date is fully specified by the year (numbered by some scheme beyond the scope of the calendar itself), the month (identified by name or number), and the day of the month (numbered sequentially starting at 1).

Although the calendar year runs from 1 January to 31 December, at previous times year numbers were based on a different starting point within the calendar. (See the History section below.)

Lunar calendar

Main article: Computus

The Catholic Church maintained a tabular lunar calendar, which was primarily to calculate the date of Easter, and the lunar calendar required reform as well. A perpetual lunar calendar was created, in the sense that 30 different arrangements (lines in the expanded table of epacts) for lunar months were created. One of the 30 arrangements applies to a century (for this purpose, the century begins with a year divisible by 100). When the arrangement to be used for a given century is communicated, anyone in possession of the tables can find the age of the moon on any date, and calculate the date of Easter.

History

Gregorian reform

Pope Gregory XIII celebrating the introduction of the Gregorian calendar. Detail of the pope's tomb by Camillo Rusconi

The calendar was a reform in 1582 to the Julian calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by papal bull Inter gravissimas dated 24 February 1582. The motivation for the adjustment was to bring the date for the celebration of Easter to the time of the year in which the First Council of Nicaea had agreed upon in 325. Although a canon of the council specified that all Christians should celebrate Easter on the same day, it took almost five centuries before virtually all Christians achieved that objective by adopting the rules of the Church of Alexandria (see Easter for the issues which arose).

The Council of Trent approved a plan in 1563 for correcting the calendrical errors, requiring that the date of the vernal equinox be restored to that which it held at the time of the First Council of Nicaea in 325 and that an alteration to the calendar be designed to prevent future drift. This would allow for a more consistent and accurate scheduling of the feast of Easter. In 1577, a Compendium was sent to expert mathematicians outside the reform commission for comments. Some of these experts, including Giambattista Benedetti and Giuseppe Moleto, believed Easter should be computed from the true motions of the sun and moon, rather than using a tabular method, but these recommendations were not adopted. The reform adopted was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio). Lilius's proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years: this part of the proposal had been suggested before by, among others, Pietro Pitati. Lilius also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.

Lilius's proposals had two components. Firstly, he proposed a correction to the length of the year. The mean tropical year is 365.24219 days long, while the mean vernal equinox year is 365.2424 days. As the average length of a Julian year is 365.25 days, the Julian year is almost 11 minutes longer than the mean year. The discrepancy results in a drift of about three days every 400 years. Lilius's proposal resulted in an average year of 365.2425 days. (See Accuracy). At the time of Gregory's reform there had already been a drift of 10 days since the Council of Nicaea, resulting in the vernal equinox falling on 11 March instead of the ecclesiastically fixed date of 21 March, and if unreformed it would drift further. Lilius proposed that the 10-day drift should be corrected by deleting the Julian leap day on each of its ten occurrences over a period of 40 years, thereby providing for a gradual return of the equinox to 21 March. Lilius's work was expanded upon by Christopher Clavius in a closely argued, 800-page volume. He would later defend his and Lilius's work against detractors. Clavius's opinion was that the correction should take place in one move, and it was this advice which prevailed with Gregory.

The second component consisted of an approximation which would provide an accurate yet simple, rule-based calendar. Lilius's formula was a 10-day correction to revert the drift since the Council of Nicaea, and the imposition of a leap day in only 97 years in 400 rather than in 1 year in 4. The proposed rule was that years divisible by 100 would be leap years only if they were divisible by 400 as well.

Because the spring equinox was tied to the date of Easter, the Catholic Church considered the seasonal drift in the date of Easter undesirable. The Church of Alexandria celebrated Easter on the Sunday after the 14th day of the moon (computed using the Metonic cycle) that falls on or after the vernal equinox, which they placed on 21 March. However, the Church of Rome still regarded 25 March as the equinox (until 342) and used a different cycle to compute the day of the moon. In the Alexandrian system, since the 14th day of the Easter moon could fall at earliest on 21 March its first day could fall no earlier than 8 March and no later than 5 April. This meant that Easter varied between 22 March and 25 April. In Rome, Easter was not allowed to fall later than 21 April, that being the day of the Parilia or birthday of Rome and a pagan festival.The first day of the Easter moon could fall no earlier than 5 March and no later than 2 April.

Easter was the Sunday after the 15th day of this moon, whose 14th day was allowed to precede the equinox. Where the two systems produced different dates there was generally a compromise so that both churches were able to celebrate on the same day. By the 10th century all churches (except some on the eastern border of the Byzantine Empire) had adopted the Alexandrian Easter, which still placed the vernal equinox on 21 March, although Bede had already noted its drift in 725—it had drifted even further by the 16th century.

Worse, the reckoned Moon that was used to compute Easter was fixed to the Julian year by a 19-year cycle. However, that approximation built up an error of one day every 310 years, so by the 16th century the lunar calendar was out of phase with the real Moon by four days.

The 19-year cycle used for the lunar calendar was also to be corrected by one day every 300 or 400 years (8 times in 2500 years) along with corrections for the years that are no longer leap years (i.e., 1700, 1800, 1900, 2100, etc.). In fact, a new method for computing the date of Easter was introduced.

Gregory dropped 10 days to bring the calendar back into synchronization with the seasons. Accordingly, when the new calendar was put in use, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of ten days. The last day of the Julian calendar was Thursday, 4 October 1582 and this was followed by the first day of the Gregorian calendar, Friday, 15 October 1582 (the cycle of weekdays was not affected).

Adoption

Although Gregory's reform was enacted in the most solemn of forms available to the Church, the bull had no authority beyond the Catholic Church and the Papal States. The changes that he was proposing were changes to the civil calendar, over which he had no authority. They required adoption by the civil authorities in each country to have legal effect.

The bull Inter gravissimas became the law of the Catholic Church in 1582, but it was not recognised by Protestant Churches, Orthodox Churches, and a few others. Consequently, the days on which Easter and related holidays were celebrated by different Christian Churches again diverged.

Adoption in Europe

Four Catholic countries - Spain, Portugal, the Polish–Lithuanian Commonwealth, and most of Italy - implemented the new calendar on the date specified by the bull, with Julian Thursday, 4 October 1582 being followed by Gregorian Friday, 15 October 1582. The Spanish and Portuguese colonies adopted the calendar later because of the slowness of communication. Other Catholic countries soon followed. France adopted the new calendar with Sunday, 9 December 1582 being followed by Monday, 20 December 1582. The Dutch provinces of Brabant, Zeeland and the Staten-Generaal also adopted it on 25 December of that year, the provinces forming the Southern Netherlands (modern Belgium) except the Duchy of Brabant on 1 January 1583, and the province of Holland followed suit on 12 January 1583.

Many Protestant countries initially objected to adopting a Catholic invention; some Protestants feared the new calendar was part of a plot to return them to the Catholic fold. In the Czech lands, Protestants resisted the calendar imposed by the Habsburg Monarchy. In parts of Ireland, Catholic rebels until their defeat in the Nine Years' War kept the "new" Easter in defiance of the English-loyal authorities; later, Catholics practising in secret petitioned the Propaganda Fide for dispensation from observing the new calendar, as it signaled their disloyalty.

In the 1690s, Ole Rømer used his influence to bring about a uniform adoption of the Gregorian calendar in Protestant countries, though that could not be achieved in practice. Denmark, which then included Norway and some Protestant states of Germany, adopted the solar portion of the new calendar with Sunday, 18 February 1700 being followed by Monday, 1 March 1700. Prussia adopted the Gregorian calendar on the same day. However, Denmark did not adopt the lunar portion, but decided to calculate the date of Easter astronomically using the instant of the vernal equinox and the full moon according to Kepler's Rudolphine Tables of 1627. They finally adopted the lunar portion of the Gregorian calendar in 1776. The remaining provinces of the Dutch Republic also adopted the Gregorian calendar in July 1700 (Gelderland), December 1700 (Utrecht and Overijssel) and January 1701 (Friesland and Groningen).

Sweden's relationship with the Gregorian Calendar was a difficult one. Sweden started to make the change from the Julian calendar and towards the Gregorian calendar in 1700, but it was decided to make the (then 11-day) adjustment gradually, by excluding the leap days (29 February) from each of 11 successive leap years, 1700 to 1740. In the meantime, the Swedish calendar would be out of step with both the Julian calendar and the Gregorian calendar for 40 years; also, the difference would not be constant but would change every 4 years. This system had potential for confusion when working out the dates of Swedish events in this 40-year period. To add to the confusion, the system was poorly administered and the leap days that should have been excluded from 1704 and 1708 were not excluded. The Swedish calendar (according to the transition plan) should now have been 8 days behind the Gregorian, but was still in fact 10 days behind. King Charles XII recognised that the gradual change to the new system was not working, and he abandoned it.

However, rather than proceeding directly to the Gregorian calendar, it was decided to revert to the Julian calendar. This was achieved by introducing the unique date 30 February in the year 1712, adjusting the discrepancy in the calendars from 10 back to 11 days. Sweden finally adopted the Gregorian calendar in 1753, when Wednesday, 17 February was followed by Thursday, 1 March. Since Finland was under Swedish rule at that time, it did the same. Finland's annexation to the Russian Empire did not revert this, since autonomy was granted, but government documents in Finland were dated in both the Julian and Gregorian styles. This practice ended when independence was gained in 1917.

William Hogarth painting: Humours of an Election (c. 1755), which is the main source for "Give us our Eleven Days".

Britain and the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752, by which time it was necessary to correct by 11 days. Wednesday, 2 September 1752 was followed by Thursday, 14 September 1752. Claims that rioters demanded "Give us our eleven days" grew out of a misinterpretation of a painting by William Hogarth. After 1753, the British tax year in Britain continued to operate on the Julian calendar and began on 5 April, which was the "Old Style" new tax year of 25 March. A 12th skipped Julian leap day in 1800 changed its start to 6 April. It was not changed when a 13th Julian leap day was skipped in 1900, so the tax year in the United Kingdom still begins on 6 April.

In Alaska, the change took place when Friday, 6 October 1867 was followed again by Friday, 18 October after the US purchase of Alaska from Russia, which was still on the Julian calendar. Instead of 12 days, only 11 were skipped, and the day of the week was repeated on successive days, because the International Date Line (although not known by that name in 1867) was shifted from Alaska's eastern to western boundary along with the change to the Gregorian calendar.

In Russia the Gregorian calendar was accepted after the October Revolution (so named because it took place in October 1917 in the Julian calendar). On 24 January 1918 the Council of People's Commissars issued a Decree that Wednesday, 31 January 1918 was to be followed by Thursday, 14 February 1918, thus dropping 13 days from the calendar.

The other countries of Eastern Europe, most notably Orthodox countries, adopted the Gregorian calendar in the 1910s or early 1920s. The last country of Eastern Orthodox Europe to adopt the Gregorian calendar was Greece with Wednesday, 15 February 1923 being followed by Thursday, 1 March 1923.

As European powers colonized or otherwise took control of various parts of the world, they generally applied their calendars in those countries. Those countries have generally retained the Gregorian calendars after achieving independence.

Adoption in Eastern Asia

Japan decided to officially replace its traditional lunisolar calendar with the Gregorian calendar in 1872, so the day following the second day of the twelfth month of the fifth year of the Meiji emperor's rule, became 1 January 1873, thus bringing Japan's calendar in alignment with that of the major Western powers (excluding Russia). Still, it is common to use Nengo, reign names instead of the Common Era or Anno Domini system, especially for official documents; for instance, Meiji 1=1868, Taisho 1=1912, Showa 1=1926, Heisei 1=1989, and so on. Still, this system has increasingly been replaced in popular usage by the "Western calendar" (西暦, seireki) over the course of the twentieth century.

Korea adopted the Gregorian calendar on 1 January 1895 with the active participation of Yu Kil-chun. Although the new calendar continued to number its months, there were multiple systems used to refer to its years: during the Joseon Dynasty, in 1895–97, its years were numbered from the founding of that dynasty, regarding 1392 as year one; then between 1897 and 1910, and again from 1948 to 1962, Korean era names were used for its years; and between 1910 and 1945, when Korea was under Japanese rule, Japanese era names were used to count the years of the Gregorian calendar used in Korea.

In South Korea, from 1945 until 1961, Gregorian calendar years were also counted from the foundation of Gojoseon in 2333 BC (regarded as year one), the date of the legendary founding of Korea by Dangun, hence these Dangi (단기) years were 4278 to 4294. This numbering was informally used with the Korean lunar calendar before 1945 but is only occasionally used today. In North Korea, the Juche calendar has been used since 1997 to number its years, regarding the year of birth of Kim Il Sung in 1912 as year one.

The Republic of China (ROC) formally adopted the Gregorian calendar at its founding on 1 January 1912, but China soon descended into a period of warlordism with different warlords using different calendars. With the unification of China under the Kuomintang in October 1928, the Nationalist Government decreed that effective 1 January 1929 the Gregorian calendar would be used. However, China retained the Chinese traditions of numbering the months and a modified Era System, backdating the first year of the ROC to 1912; this system is still in use in Taiwan where the ROC government retains control. Upon its foundation in 1949, the People's Republic of China continued to use the Gregorian calendar with numbered months, but abolished the ROC Era System and adopted Western numbered years.

Adoption by Orthodox Churches

Greece was the last Orthodox country to adopt the Gregorian calendar in March 1923. However, these adoptions are only for civil or state purposes, and none of the national Orthodox Churches have recognised the Gregorian calendar for church or religious purposes. Instead, a Revised Julian calendar was proposed in May 1923 which dropped 13 days in 1923 and adopted a different leap year rule. There will be no difference between the two calendars until 2800. The Orthodox churches of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, and Bulgaria adopted the Revised Julian calendar, so until 2800 these New calendarists would celebrate Christmas on 25 December in the Gregorian calendar, the same day as the Western churches. The Armenian Apostolic Church adopted the Gregorian calendar in 1923, except in the Armenian Patriarchate of Jerusalem, where the old Julian calendar is still in use.

The Orthodox churches of Jerusalem, Russia, Serbia, Georgia, Poland, Macedonia and the Greek Old Calendarists did not accept the Revised Julian calendar, and continue to celebrate Christmas on 25 December in the Julian calendar, which is 7 January in the Gregorian calendar until 2100. The refusal to accept the Gregorian reforms also has an impact on the date of Easter. This is because the date of Easter is determined with reference to 21 March as the functional equinox, which continues to apply in the Julian calendar, even though the civil calendar in the native countries now use the Gregorian calendar.

All of the other Eastern churches, the Oriental Orthodox churches (Coptic, Ethiopian, Eritrean, and Syrian) continue to use their own calendars, which usually result in fixed dates being celebrated in accordance with the Julian calendar. The Indian Orthodox Church uses the Gregorian Calendar.

The Assyrian Church uses the Gregorian Calendar as enacted by Mar Dinkha, causing a schism; the Ancient Assyrian Church of the East continues to use the Julian Calendar.

All Eastern churches continue to use the Julian Easter with the sole exception of the Finnish Orthodox Church, which has adopted the Gregorian Easter.

See also: Coptic calendar and Ethiopian calendar

Present situation

Today, a vast majority of countries use the Gregorian calendar as their sole civil calendar. Countries which have not adopted the Gregorian calendar include Saudi Arabia, Ethiopia, Nepal, Iran and Afghanistan. Some countries use other calendars alongside the Gregorian calendar, including India, Bangladesh, Israel and Myanmar; and other countries use a modified version of the Gregorian calendar, including Sri Lanka, Cambodia, Thailand, Japan, North Korea and Taiwan. Greece was the last European nation to adopt it for civil purposes (1923).

The Gregorian calendar has not been adopted by many religious organizations, which have often retained their own calendars. Alternate calendars are used in many regions of the world today to mark cycles of religious and astrological events.

Timeline

The date when each country adopted the Gregorian calendar, or an equivalent, is marked against a horizontal time line. The vertical axis is used for expansion to show separate national names for ease in charting, but otherwise has no significance.

Difference between Gregorian and Julian calendar dates

Since the introduction of the Gregorian calendar, the difference between Gregorian and Julian calendar dates has increased by three days every four centuries:

Gregorian range	Julian range	Difference
From 15 October 1582 to 10 March 1700	From 5 October 1582 to 28 February 1700	10 days
From 11 March 1700 to 11 March 1800	From 29 February 1700 to 28 February 1800	11 days
From 12 March 1800 to 12 March 1900	From 29 February 1800 to 28 February 1900	12 days
From 13 March 1900 to 13 March 2100	From 29 February 1900 to 28 February 2100	13 days
From 14 March 2100 to 14 March 2200	From 29 February 2100 to 28 February 2200	14 days

A more extensive list is available at Conversion between Julian and Gregorian calendars.

This section always places the intercalary day on 29 February even though it was always obtained by doubling 24 February (the bissextum (twice sixth) or bissextile day) until the late Middle Ages. The Gregorian calendar is proleptic before 1582 (assumed to exist before 1582) while the Julian calendar is proleptic before year AD 1 (because non-quadrennial leap days were used between 45 BC and AD 1).

The following equation gives the number of days (actually, dates) that the Gregorian calendar is ahead of the Julian calendar, called the secular difference between the two calendars. A negative difference means the Julian calendar is ahead of the Gregorian calendar.

$D=H-{\tfrac {H}{4}}-2$

where $D$ is the secular difference; $H$ is the hundreds digits of the year using astronomical year numbering, that is, use (year BC) − 1 for BC years; and ${\tfrac {H}{4}}$ is integer division of H by 4. This truncates (removes) any decimal fraction (remainder) of the division, for both positive and negative numbers. Thus during the 1900s, 19/4 = 4, while during the −500s, −5/4 = −1.

The calculated difference increases by one in a centurial year (a year ending in '00) at either 29 February Julian or 1 March Gregorian, whichever is later. For positive differences, 29 February Julian is later, whereas for negative differences, 1 March Gregorian is later.

Beginning of the year

The year used in dates during the Roman Republic and the Roman Empire was the consular year, which began on the day when consuls first entered office—probably 1 May before 222 BC, 15 March from 222 BC and 1 January from 153 BC. The Julian calendar, which began in 45 BC, continued to use 1 January as the first day of the new year. Even though the year used for dates changed, the civil year always displayed its months in the order January through December from the Roman Republican period until the present.

During the Middle Ages, under the influence of the Christian Church, many Western European countries moved the start of the year to one of several important Christian festivals—25 December (the Nativity of Jesus), 25 March (Annunciation), or Easter (France), while the Byzantine Empire began its year on 1 September and Russia did so on 1 March until 1492 when the year was moved to 1 September.

In common usage, 1 January was regarded as New Year's Day and celebrated as such, but from the 12th century until 1751 the legal year in England began on 25 March (Lady Day). So, for example, the Parliamentary record lists the execution of Charles I on 30 January as occurring in 1648 (as the year did not end until 24 March), although modern histories adjust the start of the year to 1 January and record the execution as occurring in 1649.

Most Western European countries changed the start of the year to 1 January before they adopted the Gregorian calendar. For example, Scotland changed the start of the Scottish New Year to 1 January in 1600 (this means that 1599 was a short year). England, Ireland and the British colonies changed the start of the year to 1 January in 1752 (so 1751 was a short year with only 282 days) though the tax year has stayed as 25 March to this day (being 6 April in the new calendar). Later that year in September the Gregorian calendar was introduced throughout Britain and the British colonies (see the section Adoption). These two reforms were implemented by the Calendar (New Style) Act 1750.

In some countries, an official decree or law specified that the start of the year should be 1 January. For such countries we can identify a specific year when a 1 January-year became the norm. But in other countries the customs varied, and the start of the year moved back and forth as fashion and influence from other countries dictated various customs.

Country	Start numbered year on 1 January	Adoption of Gregorian calendar
Denmark	Gradual change from 13th to 16th centuries	1700
Venice	1522	1582
Holy Roman Empire (Catholic states)	1544	1583
Spain	1556	1582
Portugal	1556	1582
Holy Roman Empire (Protestant states)	1559	1700
Sweden	1559	1753
France	1564	1582
Southern Netherlands	1576	1582
Lorraine	1579	1682
Dutch Republic	1583	1582
Scotland	1600	1752
Russia	1700	1918
Tuscany	1721	1750
Britain and British Empire except Scotland	1752	1752

Neither the papal bull nor its attached canons explicitly fix such a date, though it is implied by two tables of saint's days, one labelled 1582 which ends on 31 December, and another for any full year that begins on 1 January. It also specifies its epact relative to 1 January, in contrast with the Julian calendar, which specified it relative to 22 March. These would have been the inevitable result of the above shift in the beginning of the Julian year.

Dual dating

Main article: Dual dating

During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate the date of some event in both the Julian calendar and in the Gregorian calendar, for example, "10/21 February 1750/51", where the dual year accounts for some countries already beginning their numbered year on 1 January while others were still using some other date. Even before 1582, the year sometimes had to be double dated because of the different beginnings of the year in various countries. Woolley, writing in his biography of John Dee (1527–1608/9), notes that immediately after 1582 English letter writers "customarily" used "two dates" on their letters, one OS and one NS.

Old Style and New Style dates

Main article: Old Style and New Style dates

"Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which system is used in the British Empire and other countries that did not immediately change. Because the Calendar Act of 1750 altered the start of the year, and also aligned the British calendar with the Gregorian calendar, there is some confusion as to what these terms mean. They can indicate that the start of the Julian year has been adjusted to start on 1 January (NS) even though contemporary documents use a different start of year (OS); or to indicate that a date conforms to the Julian calendar (OS), formerly in use in many countries, rather than the Gregorian calendar (NS).

Proleptic Gregorian calendar

The Gregorian calendar can, for certain purposes, be extended backwards to dates preceding its official introduction, producing the proleptic Gregorian calendar, but this proleptic calendar should be used with great caution.

For ordinary purposes, the dates of events occurring prior to 15 October 1582 are generally shown as they appeared in the Julian calendar, with the year starting on 1 January, and no conversion to their Gregorian equivalents. The Battle of Agincourt is universally known to have been fought on 25 October 1415 which is Saint Crispin's Day.

Usually, the mapping of new dates onto old dates with a start of year adjustment works well with little confusion for events that happened before the introduction of the Gregorian Calendar. But for the period between the first introduction of the Gregorian calendar on 15 October 1582 and its introduction in Britain on 14 September 1752, there can be considerable confusion between events in continental western Europe and in British domains in English language histories.

Events in continental western Europe are usually reported in English language histories as happening under the Gregorian calendar. For example the Battle of Blenheim is always given as 13 August 1704. Confusion occurs when an event affects both. For example William III of England arrived at Brixham in England on 5 November 1688 (Julian calendar), after setting sail from the Netherlands on 11 November 1688 (Gregorian calendar).

Shakespeare and Cervantes apparently died on exactly the same date (23 April 1616), but Cervantes predeceased Shakespeare by ten days in real time (as Spain used the Gregorian calendar, but Britain used the Julian calendar). This coincidence encouraged UNESCO to make 23 April the World Book and Copyright Day.

Astronomers avoid this ambiguity by the use of the Julian day number.

For dates before the year 1, unlike the proleptic Gregorian calendar used in the international standard ISO 8601, the traditional proleptic Gregorian calendar (like the Julian calendar) does not have a year 0 and instead uses the ordinal numbers 1, 2, … both for years AD and BC. Thus the traditional time line is 2 BC, 1 BC, AD 1, and AD 2. ISO 8601 uses astronomical year numbering which includes a year 0 and negative numbers before it. Thus the ISO 8601 time line is −0001, 0000, 0001, and 0002.

Months of the year

English speakers sometimes remember the number of days in each month by memorizing a traditional mnemonic verse:

Thirty days hath September,

April, June, and November.

All the rest have thirty-one,

Excepting February alone,

Which hath twenty-eight days clear,

And twenty-nine in each leap year.

For variations and alternate endings, see Thirty days hath September.

The knuckle mnemonic.

A language-independent alternative used in many countries is to hold up one's two fists with the index knuckle of the left hand against the index knuckle of the right hand. Then, starting with January from the little knuckle of the left hand, count knuckle, space, knuckle, space through the months. A knuckle represents a month of 31 days, and a space represents a short month (a 28- or 29-day February or any 30-day month). The junction between the hands is not counted, so the two index knuckles represent July and August.

This method also works by starting the sequence on the right hand's little knuckle, then continuing towards the left. It can also be done using just one hand: after counting the fourth knuckle as July, start again counting the first knuckle as August. A similar mnemonic can be found on a piano keyboard: starting on the key F for January, moving up the keyboard in semitones, the black notes give the short months, the white notes the long ones.

The origins of English naming used by the Gregorian calendar:

• January: Janus (Roman god of gates, doorways, beginnings and endings)
• February: Februus (Etruscan god of death) Februarius (mensis) (Latin for "month of purification (rituals)" it is said to be a Sabine word, the last month of ancient pre-450 BC Roman calendar). It is related to fever.
• March: Mars (Roman god of war)
• April: "Modern scholars associate the name with an ancient root meaning 'other', i.e the second month of a year beginning in March."
• May: Maia Maiestas (Roman goddess of springtime, warmth, and increase)
• June: Juno (Roman goddess, wife of Jupiter)
• July: Julius Caesar (Roman dictator) (month was formerly named Quintilis, the fifth month of the calendar of Romulus)
• August: Augustus (first Roman emperor) (month was formerly named Sextilis, the sixth month of Romulus)
• September: septem (Latin for seven, the seventh month of Romulus)
• October: octo (Latin for eight, the eighth month of Romulus)
• November: novem (Latin for nine, the ninth month of Romulus)
• December: decem (Latin for ten, the tenth month of Romulus)

Week

Main article: Seven-day week

In conjunction with the system of months there is a system of weeks. A physical or electronic calendar provides conversion from a given date to the weekday, and shows multiple dates for a given weekday and month. Calculating the day of the week is not very simple, because of the irregularities in the Gregorian system. When the Gregorian calendar was adopted by each country (but not Alaska), the weekly cycle continued uninterrupted. For example, in the case of the few countries that adopted the reformed calendar on the date proposed by Gregory XIII for the calendar's adoption, Friday, 15 October 1582, the preceding date was Thursday, 4 October 1582 (Julian calendar).

Opinions vary about the numbering of the days of the week. ISO 8601, in common use worldwide, starts with Monday=1; printed monthly calendar grids list Mondays in the first (left) column of dates and Sundays in the last. Software often starts with Sunday=0, which places Sundays in the left column of a monthly calendar page.

Accuracy

The Gregorian calendar improves the approximation made by the Julian calendar by skipping three Julian leap days in every 400 years, giving an average year of 365.2425 mean solar days long. This approximation has an error of about one day per 3,300 years with respect to the mean tropical year. However, because of the precession of the equinoxes, the error with respect to the vernal equinox (which occurs, on average, 365.24237 days apart near 2000) is 1 day every 7,700 years. By any criterion, the Gregorian calendar is substantially more accurate than the 1 day in 128 years error of the Julian calendar (average year 365.25 days).

In the 19th century, Sir John Herschel proposed a modification to the Gregorian calendar with 969 leap days every 4000 years, instead of 970 leap days that the Gregorian calendar would insert over the same period. This would reduce the average year to 365.24225 days. Herschel's proposal would make the year 4000, and multiples thereof, common instead of leap. While this modification has often been proposed since, it has never been officially adopted.

On time scales of thousands of years, the Gregorian calendar falls behind the astronomical seasons because the slowing down of the Earth's rotation makes each day slightly longer over time (see tidal acceleration and leap second) while the year maintains a more uniform duration. Borkowski reviewed mathematical models in the literature, and found that the results generally fall between a model by McCarthy and Babcock, and another by Stephenson and Morrison. If so, in the year 4000, the calendar will fall behind by at least 0.8 but less than 1.1 days. In the year 12,000 the calendar would fall behind by at least 8 but less than 12 days.

Calendar seasonal error

This image shows the difference between the Gregorian calendar and the astronomical seasons.

The y-axis is the date in June and the x-axis is Gregorian calendar years.

Each point is the date and time of the June Solstice on that particular year. The error shifts by about a quarter of a day per year. Centurial years are ordinary years, unless they are divisible by 400, in which case they are leap years. This causes a correction on years 1700, 1800, 1900, 2100, 2200, and 2300.

For instance, these corrections cause 23 December 1903 to be the latest December solstice, and 20 December 2096 to be the earliest solstice—2.25 days of variation compared with the seasonal event.

English names for year numbering system

Main article: Anno Domini

The Anno Domini (Latin for "in the year of the/our Lord") system of numbering years, in which the leap year rules are written, and which is generally used together with the Gregorian calendar, is also known in English as the Common Era. Years before the beginning of the era are known in English as Before Christ, or Before the Common Era. The corresponding abbreviations AD, CE, BC, and BCE are used. There is no year 0; AD 1 immediately follows 1 BC.

Naturally, since Inter gravissimas was written in Latin, it does not mandate any English language nomenclature. Two era names occur within the bull, "anno Incarnationis dominicæ" ("in the year of the Incarnation of the Lord") for the year it was signed, and "anno à Nativitate Domini nostri Jesu Christi" ("in the year from the Nativity of our Lord Jesus Christ") for the year it was printed. Nevertheless, "anno Domini" and its inflections "anni Domini" and "annus Domini" are used many times in the canons attached to the bull.

Proposed reforms

The following are proposed reforms of the Gregorian calendar:

• Holocene calendar
• International Fixed Calendar (also called the International Perpetual calendar)
• World Calendar
• World Season Calendar
• Leap week calendars
  • Pax Calendar
  • Common-Civil-Calendar-and-Time
  • Symmetry454
  • Hanke-Henry Permanent Calendar

See also

• Calendar reform
• Computus — Gregorian lunar calendar
• Doomsday rule
• Dual dating
• Inter gravissimas in English — Wikisource
• Julian day calculation
• List of calendars
• Old Style and New Style dates
• Old Calendarists
  • Greek Old Calendarists

References

• Blackburn, B. & Holford-Strevens, L. (1999). The Oxford Companion to the Year. Oxford University Press. ISBN 0-19-214231-3.
• Blackburn, B. & Holford-Strevens, L. (2003). The Oxford Companion to the Year: An exploration of calendar customs and time-reckoning, Oxford University Press.
• Coyne, G. V., Hoskin, M. A., Pedersen, O. (Eds.) (1983). Gregorian Reform of the Calendar: Proceedings of the Vatican conference to commemorate its 400th anniversary, 1582–1982. Vatican City: Pontifical Academy of Sciences, Vatican Observatory (Pontificia Academia Scientarum, Specola Vaticana).
• Duncan, D. E. (1999). Calendar: Humanity's Epic Struggle To Determine A True And Accurate Year. Harper Perennial. ISBN 0-380-79324-5.
• Gregory XIII. (2002). Inter Gravissimas (W. Spenser & R. T. Crowley, Trans.). International Organization for Standardization. (Original work published 1582)
• Lee, Peter H. (Ed.) (1996). Sourcebook of Korean Civilization: Vol.2: From the seventeenth century to the modern period. Columbia University Press. ISBN 0-231-07914-1.
• Lee, P.H. & de Bary, W. T. (Eds., with Yongho Ch'oe & Kang, H. H. W.) (2000). Sources of Korean Tradition, (Vol. 2). New York: Columbia University Press.
• Meeus, J. & Savoie, D. (1992). The history of the tropical year. Journal of the British Astronomical Association, 102(1): 40–42.
• Morrison, L. V. & Stephenson, F. R. (2004). Historical values of the Earth's clock error ΔT and the calculation of eclipses. Journal for the History of Astronomy Vol. 35, Part 3, No. 120, pp. 327–336.
• Moyer, Gordon (May 1982). "The Gregorian Calendar". Scientific American, pp. 144–152.
• Moyer, Gordon (1983). "Aloisius Lilius and the Compendium Novae Rationis Restituendi Kalendarium". In Coyne, Hoskin, Pedersen (1983), pp. 171–188.
• Online Etymology Dictionary. Retrieved 23 August 2006.
• Pedersen, O. (1983). "The Ecclesiastical Calendar and the Life of the Church". In Coyne, Hoskin, Pedersen (1983), pp. 17–74.
• Seidelmann, P. K. (Ed.) (1992). Explanatory Supplement to the Astronomical Almanac. Sausalito, CA: University Science Books.
• Sumner, Charles. (1875). The cession of Russian America to the United States in The Works of Charles Sumner, vol. 11. Boston: Lea and Shepard.
• US Social Security Administration. (2005). "Gregorian/Julian Calendar" in Program Operations Manual System.

• Ziggelaar, A. (1983). "The Papal Bull of 1582 Promulgating a Reform of the Calendar". In Coyne, Hoskin, Pedersen (1983), pp. 201–239.

External links

• Gregorian calendar on In Our Time at the BBC
• Calendar Converter
• Inter Gravissimas (Latin and French plus English)
• History of Gregorian Calendar
• The Perpetual Calendar Gregorian Calendar adoption dates for many countries.
• World records for mentally calculating the day of the week in the Gregorian Calendar
• The Calendar FAQ – Frequently Asked Questions about Calendars
• Today's date (Gregorian) in over 400 more-or-less obscure foreign languages