The Gregorian calendar is the calendar currently used in the Western world. A modification of the Julian calendar, it was first proposed by Neapolitan doctor Aloysius Lilius, and promulgated by the eponymous Pope Gregory XIII on February 24, 1582 (Note: The document was dated 1581 as the year was at that time taken to commence in March).
The Gregorian calendar was devised because the mean year in the Julian Calendar was a little too long, causing the Vernal equinox to slowly drift earlier in the calendar year.
The motivation of the Catholic Church in adjusting the calendar was to have Easter celebrated at the time that had been agreed at the First Council of Nicaea in 325, ie. on the Sunday after the 14th day of the Moon that falls on or after the vernal equinox—which fell approximately on March 21 at that time. By the time of this council, the drift of the equinox since the introduction of the Julian calendar had already been noticed. Instead of modifying the calendar, the equinox was standardized at March 21 instead of the original March 24 or March 25. However by the 16th century, the equinox had drifted noticeably further.
Worse, the reckoned Moon that was used to compute Easter was fixed to the Julian year by a 19-year cycle. However, that is an approximation that built up an error of 1 day every 310 years. So by the 16th century the lunar calendar was well out of phase with the real Moon too.
The fix for the equinox was to define that years divisible by 100 will be leap years only if they are divisible by 400 as well. So, in the last millennium, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. In this millennium, 2100, 2200, 2300 and 2500 will not be leap years, but 2400 will be.
When the new calendar was put in use, to correct the error already accumulated in the thirteen centuries since the council of Nicaea, a deletion of ten days was made. The last day of the Julian calendar was October 4, 1582 and this was followed by the first day of the Gregorian calendar October 15, 1582. This created some consternation, and the church was accused of stealing ten days of people's lives. The dates "5 October 1582" to "14 October 1582" (inclusive) exist only in the Proleptic Gregorian calendar, which is confined to special scientific contexts and has no relevance for dating ordinary historical events.
From 1 January 1622, the first day of the year was standardised as January 1. This was already the system used in Italy, Germany, and other places, but not universally (England, for example, began the year on March 25).
It is also sometimes necessary to indicate that the year itself had two different designations because of the change to the beginning of the year, for example, "February 10/February 21, 1751/1752". This confusion pre-dated the change to the calendar because the Church and the State had always used different systems for different purposes.
The 19-year cycle used for the lunar calendar was also to be corrected by 1 day every 300 or 400 years (8 times in 2500 years) along with corrections for the years (1700, 1800, 1900, 2100 etc.) that are no longer leap years. In fact, a new method for computing the date of Easter was introduced.
Adoption outside of Roman Catholic nations
Very few countries accepted the new calendar immediately. Non-Catholic countries objected to adopting a Catholic invention. England, Scotland and thereby the rest of the British Empire (including part of what is now the United States) did not adopt it until 1752, by which time it was necessary to correct by eleven days (September 2, 1752 being followed by September 14, 1752). Again, people objected to the change; although in this case it was not because they literally thought days were being stolen from their lives. It was because they were required to pay a full month's rent for the shortened September but they were paid only for the days actually worked, and the loss of income caused financial hardship.
Sweden's relationship with the Gregorian Calendar had a difficult birth. Sweden started to make the change from the OS calendar and towards the NS calendar in 1700, but it was decided to make the now 11-day adjustment gradually, by excluding the leap days (29 February) from each of 11 successive leap years, 1700 to 1740. In the meantime, not only would the Swedish calendar be out of step with both the Julian calendar and the Gregorian calendar for 40 years, but also the difference would not be static but would change every 4 years. This strange system clearly had great potential for endless confusion when working out on what dates events in Sweden actually occurred in this period. To make matters worse, the system was poorly administered and the leap days that should have been excluded from 1704 and 1708 were still for some reason included. The Swedish calendar should by now have been 8 days behind the Gregorian, but it was still in fact 10 days behind. King Charles XII wisely recognised that the gradual change to the new system was not working and he abandoned it. However, rather than now proceeding directly to the Gregorian calendar (as in hindsight seems to have been the sensible and obvious thing to do), it was decided to revert to the Julian calendar. This was achieved by introducing the unique date February 30 in the year 1712, adjusting the discrepancy in the calendars from 10 back to 11 days. Sweden finally adopted the Gregorian calendar in a sensible fashion in 1753, when February 17 was followed by March 1.
Greece followed suit in 1924. The majority of Eastern Orthodoxy did not accept the change to the new calendar for liturgical purposes, regardless of the new civil date. This includes the Orthodox Church of Russia, which maintains the Julian calendar for religious purposes while accepting the use of the Gregorian for purely secular purposes. Some Orthodox Christians may go so far as to identify themselves as Old Calendarist and assert that under the Julian Calendar the eternal liturgy in Heaven was reflected on earth by the liturgical calendar and that the change meant that Heaven and Earth would be out of tune. However, most recognize that an ecclesiastic calendar need not be identical to the civil calendar.
The Republic of China government formally adopted the system on its founding on January 1, 1912 and used nationally after it succeeded the Qing Empire. However, the ROC retained the Chinese tradition of the Era System (ie: 1912 = Year 1 of the Republic of China). China did not adopt the Common Era System until the founding of the People's Republic of China in October 1949.
Japan replaced the traditional lunisolar calendar with a solar calendar compatible to the Gregorian Calendar in 1873, but the Common Era has not been officially adopted. Official representation of years is based on imperial eras (Meiji; M1=AD1867, Taisho; T1=AD1912, Showa; S1=AD1926, Heisei; H1=AD1989, and so on future). The Common Era (seireki) is nonetheless widely accepted by civilians and to a less extent by government agencies.
Proleptic Gregorian calendar
The Gregorian calendar can for certain purposes be extended to dates preceding its official introduction, producing the Proleptic Gregorian Calendar. However this proleptic calendar should be used with great caution.
For ordinary purposes, the dates of events occurring prior to 15 October 1582 should be shown as they appeared in the Julian calendar, and not converted into their Gregorian equivalents.
However, events occurring in countries where the Gregorian calendar was introduced later than 15 October 1582 are a little more contentious. For example, in Great Britain and its overseas possessions (then including the American colonies), the new calendar was not introduced until 14 September 1752. How, then, should we date events occurring in Britain and her possessions in the 170 years between 1582 and 1752? The answer depends very much on the context, but in all cases the writer should make it absolutely clear which calendar is being used. It would be absurd to go back and change all historical records in Britain deriving from this period; however, it is often highly desirable to translate particular Old Style dates into their New Style equivalents, such as where the context includes reference to other countries that had already converted to New Style before Britain did. If comparison of dates is done using different calendars, we can encounter logical absurdities such as William and Mary of Orange seeming to arrive in London to accept the English crown, a week or so before they left the Netherlands; and Shakespeare and Cervantes apparently dying on exactly the same date, when in fact Cervantes predeceased Shakespeare by 10 days in real time. The Proleptic Gregorian calendar makes these kinds of historical comparisons meaningful.
Confusion with British vs. American usage
Dates of events in Great Britain prior to 1752 are usually now shown in their original Old Style form, whereas dates of notable events in (then British) America prior to 1752 are usually now shown in the New Style form.
- For example, Shakespeare died on 23 April (OS), and it is rare to see this converted to 3 May (NS). But while George Washington was born on 11 February (OS), his birthday is now celebrated on 22 February (NS).
However, neither of these practices is universal in either country, so it is sometimes very unclear which calendar is being used, and this can lead to false assumptions, which can lead to dates being inaccurately converted from one calendar to the other. Since the resurgence of interest in the history of the calendar, more information about the real dates of events has been forthcoming and many previous errors have been corrected. While these changes are welcome, there is still much scope for confusion.
It is therefore incumbent upon those who refer to dates in transitional periods to make it clear which calendar is being used; and if the writer does not know, they should say so.
Months of the year
The Gregorian calendar's year is divided into 12 months:
|2||February||28 or 29|
The Gregorian calendar improves the approximation made by the Julian calendar by skipping 3 Julian leap days in every 400 years, giving an average year of 365.2425 mean solar days long, which has an error of about 1 day per 3000 years with respect to the mean tropical year of 365.2422 days but less than half this error with respect to the vernal equinox tropical year of 365.2424 days.
This is substantially more accurate than the 1 day in 128 years error of the Julian calendar.
Also on any timescale over 3000 years it is expected that changes in the Earth's orbit and unpredictable rotation make it improbable that long term accuracy can be gained by any rule change requiring further regular skipping of Julian leap days.
Calendar seasonal error
This image shows the difference between the Gregorian calendar and the seasons.
The Y axis is "days error" and the X axis is Gregorian calendar years.
Each point represents a single date on a given year. The error shifts by about 1/4 day per year. Years that are multiples of 100 but not 400 are NOT leap years. This causes a correction on years 1700, 1800, 1900, 2100, 2200, and 2300.
For instance, these corrects cause December 23, 1903 to be the latest December solstice, and December 20, 2096 to be the earliest solstice, nearly 2.5 days variations with the seasonal event.
An average year is 365.2425 days = 52.1775 weeks, 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds.
A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.
A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.
(Some years may also contain a leap second.)
The 400-year cycle of the Gregorian calendar has 146097 days and hence exactly 20871 weeks. So for example the days of the week in Gregorian 1603 were exactly the same as for 2003. This also causes more months to begin on a Sunday (and hence have Friday 13) than any other day of the week. 688 out of every 4800 months (or 172/1200) begin on a Sunday, while only 684 out of every 4800 months (171/1200) begin on each of Saturday and Monday, the least common cases.
A smaller cycle is 28 years (1461 weeks), provided that there is no dropped leap year in between. Days of the week in years may also repeat after 6, 11, 12, 28 or 40 years. Intervals of 6 and 11 are only possible with common years, while intervals of 28 and 40 are only possible with leap years. A interval of 12 years can occur with either type, but only when there is a dropped leap year in between.
An algorithm called the Doomsday algorithm is a method by which you can discern which of the 14 calendar variations should be used in any given year (after the Gregorian reformation). It is based on the last day in February, referred to as the Doomsday.
- Inter Gravissimas, Gregory XIII's bull introducing the new calendar (Latin and French)
- Inter Gravissimas (Latin and French plus English)
- British Calendar Act 1751
- The Julian and Gregorian Calendars
- Frequently Asked Questions about Calendars
- The Perpetual Calendar This provides dates of changing to Gregorian Calendar for many countries.