The Gregorian calendar , also known as the civil calendar , is the most widely used calendar in the world . It was created at the end of the 16th century through a reform of the Julian calendar . It is named after Pope Gregory XIII. who decreed it in 1582 with the papal bull Inter gravissimas . Like the Julian calendar, the Gregorian calendar is a solar calendar ( solar calendar ) with an improved leap year rule (→ intercalation ) compared to the Julian calendar . It is based on an average year length of 365.2425 days, which is closer to the 365.2422 days of the solar year ( tropical year ) than the 365.25 days of the Julian calendar. The Gregorian calendar replaced both the Julian and numerous other calendars over the course of time. The date representation according to ISO 8601 is also based on the Gregorian calendar .
The essence of the Gregorian calendar reform was to prevent the calendar and solar years from drifting further apart and to better synchronize the two.
Gregorian calendar reform
The reason for the Gregorian calendar reform was not only the Julian calendar year, which was too long compared to the solar year, but also the increasing incorrect dating of the Christian Easter festival . The Julian calendar lagged the course of the year of the sun by ten days in the 16th century compared to the 4th century. The necessary failure of ten calendar days, which was arranged in one piece, caused general irritation and also led to the reluctant adoption of the Gregorian calendar within the Catholic Church . The churches that emerged from the Reformation delayed adoption for ideological reasons, because the reform had come from the Pope.
Deficiencies in the Julian calendar
Since a Julian calendar year with an average of 365.25 days is about eleven minutes longer than the solar year, the astronomical beginning of spring was postponed by one day to an earlier calendar date about every 130 years. In 1582 it fell on March 11th of the calendar corrected by the Gregorian reform, which means that the Julian calendar lagged the astronomical events in the solar year by almost two weeks. Since 19 Julian years are about 0.06 days longer than the 235 synodic months of the lunar circle , the calculated ("cyclic") time is shifted about every 16 19-year periods (ie about every 300 years) compared to the astronomical full moon time a day later in the Julian calendar. As a result, the Easter date, which is dependent on the date of the beginning of spring and the date of the spring full moon, was no longer correctly determined.
The scheme for predicting the future Easter dates entered in Easter tables (see computistics ) was fixed in the 6th century as a result of the work of Dionysius Exiguus . As early as 725 Beda established that the full moon was ahead of the calculated dates.
Since the 14th century, proposals for a calendar reform have been submitted again and again - including by Nikolaus von Kues on behalf of the Council of Basel , Regiomontanus and Nicolaus Copernicus . But these were always rejected. Nonetheless, Copernicus' work De revolutionibus orbium coelestium ("Of the revolutions of the heavenly bodies") and the Prutenic tablets by Erasmus Reinhold formed the basis for the finally published by Pope Gregory XIII. decreed reform.
Aloisius Lilius (until 1576, then his brother Antonio), Christophorus Clavius , Ignazio Danti , Pedro Chacón (1526–1581), Séraphin Olivier-Razali and Vincenzo Lauro worked in the reform commission chaired by Cardinal Guglielmo Sirleto . The commission decided to adjust the calendar so that the primary equinox would return to near March 23rd as in 46 BC. When the Julian calendar was created, and to stabilize it there by means of a more accurate mean year length.
The main component of the proposed reform was a corrected algorithm for determining the Easter festival. In addition to the corrected and future correct date of the beginning of spring, the corrected and future date of the (spring) full moon was necessary for this. The German Jesuit Christophorus Clavius, who teaches maths at the Collegio Romano in Rome, was commissioned by the Pope to work out the new calendar mathematically . He largely followed the suggestions of the physician and astronomer Aloisius Lilius .
Reform year 1582
The delay in the calendar compared to the seasons (e.g. the beginning of spring) was corrected in 1582 by omitting ten calendar days. The conditions at the time of the Council of Nicaea in 325 were restored, since resolutions on the date of Easter were first passed at this council. The beginning of spring shifted from March 23rd in Julius Caesar's time to March 21st in the 4th century. In 1583, in all countries that had immediately adopted the new (Gregorian) calendar, the beginning of spring took place again on March 21st.
When looking for a suitable time for the reform, the choice fell on October, as the calendar for this month contained comparatively few saints 'feasts and the omitted days caused only a slight disturbance of the saints' calendar . Because of the great importance of Sunday in Christianity , the reform did not interrupt the sequence of days of the week. Thursday, October 4th, was followed by Friday, October 15th. All Sundays in the Julian calendar are also Sundays in the Gregorian.
New year length (solar equation)
In order to prevent the beginning of spring from continuing to move away from March 21st, the Gregorian calendar takes into account the duration of the middle calendar year as 365.2425 instead of the previous 365.25 days. The abbreviation was made with the help of a further, superordinate switching rule, according to which those secular years (years whose number is divisible by 100 without a remainder) whose number divided by 400 does not result in a whole number are not leap years. After that, the years 1700, 1800 and 1900 were not leap years. The years 1600 and 2000 were leap years. The years 2100, 2200 and 2300 as well as 2500, 2600 and 2700 etc. will be without a leap day ( solar equation , metemptosis ). Since 117 years passed between the date correction in 1582 and 1700 when this new exception rule was first applied, the beginning of spring is on average one day early in the calendar. It commutes between March 19 and March 21, instead of symmetrically around March 21.
Correction of the lunar date (lunar equation)
To determine the Easter date, the period of the lunar circle is used, after which the phases of the moon fall on the same day every 19 solar years . So far, the small mistake made of one day in about 310 years has been ignored. During the reform, the error of about three days that had accrued was eliminated by bringing the calendar forward by three days and a more precise future correspondence was provided with the help of the lunar equation . This states that the day of the spring full moon has to be postponed one calendar day earlier every 312.5 years. Eight secular years in 2500 years are planned for this.
Correcting the length of the calendar year with omitted leap days would falsify the correction of the lunar date. Therefore, in the secular years without a leap day, the lunar date must be specified one day later in the calendar (reverse solar equation). If both the lunar and solar equations are to be applied in a secular year, the lunar date remains unchanged: - 1 day + 1 day = 0 days.
Beginning of the year
Parallel to the calendar reform, but not at the same time as it, the beginning of the year was officially postponed to January 1st ( circumcision style ), which was due to its name ( Latin ianua means "door") and the proximity to Christmas and the winter solstice as New Year's Day ; in addition, the Roman tradition was preserved in it. In the Middle Ages , the year had otherwise started on different days, including Christmas, Easter and the Annunciation ( Annunciation style ). Nevertheless, the Gregorian reform had an impact on this because the papal bull included a list of the new name days of the saints, which included the rest of the holidays from 1582 to December 31 and those of the whole of the following, newly divided year (and all future). This resulted in an overlap of eleven days (Gregorian / Julian calendar) and at the same time a year (between New Year and Easter): “On 10./21. February 1750/1751 ".
Adoption of the Gregorian calendar
Julian counterpart to /
1st day of validity of the
|New Years Day
Catholic imperial estates of the Holy Roman Empire (HRR),
(including the Archduchy of Austria and most of the Catholic places of the Swiss Confederation )
|5th jul. / October 15, 1582 greg.||1544|
|Poland-Lithuania , Kingdom of Spain , Kingdom of Portugal and their colonies||5th jul. / October 15, 1582 greg.||1556|
|Grand Duchy of Tuscany||5th jul. / October 15, 1582 greg.||1750|
|Republic of Venice||5th jul. / October 15, 1582 greg.||1797|
|Kingdom of France and colonies||10 jul. / December 20, 1582 greg.||1564|
|Southern Spanish Netherlands (area of today's Belgium)||21 jul. / December 31, 1582 greg.||1576|
|Republic of the Seven United Provinces ("Netherlands"): (Province of Holland and Province of Zeeland )||2nd jul. / January 12th 1583 greg.||1583|
|Bohemia , Canton Unterwalden||7th jul. / January 17, 1584 greg.|
|Silesia||July 13th / January 23, 1584 greg.|
|Moravia||4th jul. / October 14, 1584 greg.|
|Grand Duchy of Lithuania||December 22, 1585 Jul. / January 1, 1586 greg.||1600|
|Kingdom of Hungary||October 22nd July / November 1, 1587 greg.|
|Principality of Transylvania||15th jul. / December 25, 1590 greg.|
|Duchy of Prussia||23 August Jul. / September 2, 1612 greg.|
|Alsatian territories (each after incorporation into the Kingdom of France )||from 1648|
|Canton of Valais||1st jul. / March 11, 1656 greg.|
|Denmark-Norway , Protestant Imperial Estates of the Holy Roman Empire||February 19th Jul. / March 1, 1700 greg.||1559|
|Northeastern Netherlands ( Overijssel , Utrecht )||1st jul. / December 12th 1700 greg.||1583|
|Catholic part of the canton of Glarus||1700|
|Most of the Reformed places in Switzerland, such as Basel , Bern , Geneva , Mulhouse in Alsace , Schaffhausen and Zurich ; also provinces of Friesland and Groningen in the Netherlands||1st jul. / January 12th 1701 greg.|
|Canton of Appenzell Ausserrhoden , City of St. Gallen and reformed part of Canton Glarus||1724|
|Kingdom of Great Britain with Scotland, Wales and colonies in America||3rd jul. / 14 September 1752 greg.||1752 (Scotland 1600)|
|Kingdom of Sweden with Finland||February 18th jul. / March 1, 1753 greg.||1559|
|Duchy of Lorraine||17th jul. / February 28, 1760 greg.||1579|
|Empire of France (again after abandoning the French revolutionary calendar )||January 1, 1806 greg.|
|Last municipalities in the canton of Graubünden||1812|
|Alaska as the Department of Alaska part of the United States||1867|
|Empire of Japan||January 1, 1873 greg.|
|Latvia ( Courland under German occupation)||12 jul. / May 25, 1915 greg.|
|Lithuania under German occupation||12 jul. / May 25, 1915 greg.|
|Tsarist Bulgaria||1st jul. / April 14, 1916 greg.|
|Republic of Turkey / Ottoman Empire||February 16, Jul. / March 1, 1917 greg. *||1917|
|Latvia ( Livonia under German occupation)||23 August Jul. / September 5, 1917 greg.|
|Soviet Russia||1st jul. / February 14, 1918 greg.||1700|
|Estonia||1st jul. / February 14, 1918 greg.||1700|
|Ukrainian People's Republic||February 16, Jul. / March 1, 1918 greg.||1700|
|Transcaucasian Democratic-Federal Republic||April 18th jul. / May 1, 1918 greg.|
|Kingdom of Serbs, Croats and Slovenes °||15th jul. / January 28, 1919 greg.|
|Kingdom of Romania °||1st jul. / April 14, 1919 greg.|
|Kingdom of Greece (except Athos )||February 16, Jul. / March 1, 1923 greg.|
|People's Republic of China||° 19 1911 jul. /January 1, 1912 greg.|
Only the countries Spain , Portugal , Poland and partly Italy actually adopted the Gregorian calendar on the 5th and 15th. October 1582. Most of the Catholic countries in Europe followed in the next few years, while the Protestant countries initially rejected the new calendar decreed by the Pope. Against the background of the incipient confessionalization , this led to fierce polemics, for example the calendar dispute brought the city of Augsburg, which had been biconfessional since 1555, to the brink of civil war in 1584. Something similar happened with the calendar unrest in Riga . In the same year, most of the Catholic cantons in Switzerland introduced it ; there, on February 28th, March 11th followed. However, some Catholic Swiss territories followed later, namely Unterwalden in 1584, Valais in 1655 and Catholic Glarus in 1700.
The evangelical territories of the Holy Roman Empire did not adopt the Gregorian calendar until 1700, 118 years after its first introduction, following a resolution of the Corpus Evangelicorum . Outside the empire, the Duchy of Prussia, which was linked to the Electorate of Brandenburg in personal union, had introduced the Catholic calendar as the first Protestant territory in 1612 , under pressure from the feudal lord of the Kingdom of Poland . February 18, 1700 followed immediately in the empire. On this day, the difference between the Julian and Gregorian calendars increased from ten to eleven days, which means that if the Julian calendar is still adhered to, the conversion to the Gregorian calendar would have further complicated. Previously, for example, contracts between Catholic and Protestant princes had to be provided with both dates, for example as 3rd / 14th. April 1750 (see example on the right). At the turn of the year the year numbers differed between the areas of the old and the new calendar. The expression “ between the years ” for the days after Christmas comes from this period .
The Kingdom of Denmark , to which Norway and Iceland also belonged at the time, also introduced the Gregorian calendar from February 18 to March 1, 1700. The Reformed places in Switzerland followed almost a year later, they jumped from December 31, 1700 to January 12, 1701, with four exceptions: The Protestant half-canton of Appenzell Ausserrhoden , the Protestant city of St. Gallen and the Protestant parts of Glarus closed not until 1724, and in Graubünden the official transition to the new calendar took place between 1760 and 1812, depending on the municipality. In some areas of Switzerland, however, the old calendar remained alive for longer in the population; In the Protestant Engadin, New Year was celebrated on January 13th until around 1870, and the Appenzell New Year's Eve music still occurs on January 13th, December 31st according to the Julian calendar ( old New Year's Eve ). In England (and also in the later USA ) the Gregorian calendar was introduced on the night of September 2 to 14, 1752, in Sweden from February 17 to March 1, 1753.
The Easter calculation remained different in the Holy Roman Empire for more than 70 years, which led to different dates in the years 1724 and 1744 (1724: ev. 9, cath. 16 April; 1744: ev. 29 March, cath. 5 April) and also in 1778 and 1798. At the request of Frederick the Great , the Corpus Evangelicorum decided on December 13, 1775 to adopt an expert opinion from the Reich in which the voluntary agreement was emphasized. In avoidance of the papal name, the "Improved Imperial Calendar" was adopted. Thereupon, in agreement with all imperial estates, Emperor Joseph II determined the Gregorian calendar as the "Improved Imperial Calendar" in 1776 with formal confirmation of the expert opinion. The Protestant cantons of Switzerland, Denmark and Sweden also joined this regulation.
In Japan , where the Chinese calendar had previously applied with a few variations , the Gregorian calendar was introduced on January 1, 1873 as part of the modernization of the country. Only in the year counting does Japan use its own system to this day (see Japanese calendar ), in which the years since the accession of the reigning emperor are counted; this number is supplemented by a two-syllable motto ( nengō ) that changes from emperor to emperor .
The Orthodox countries of Eastern Europe, including Russia , kept the Julian calendar until the beginning of the 20th century. Since the years 1700, 1800 and 1900 are leap years according to the Julian calendar, the deviation from the Gregorian calendar now amounts to 13 days since 1900 and until the year 2100. The Russian October Revolution of October 25, 1917 was actually a “November Revolution ” of November 7 according to the Gregorian calendar. On this calendar day, the revolution was also celebrated until the end of the Soviet Union , after Russia introduced the new calendar calculation on February 14, 1918. Some Orthodox churches (e.g. in Russia , Serbia and Georgia ) continue to celebrate their fixed festivals according to the Julian calendar. Your Christmas (December 25th) currently falls on January 7th (Gregorian calendar). Other Orthodox churches (e.g. in Greece and Bulgaria ) use the so-called neo-Julian calendar , which will correspond to the Gregorian calendar up to the year 2799. All Orthodox churches calculate Easter and the other movable festivals after the beginning of Julian spring and after the full moon in the lunar circle; the festival therefore only occasionally coincides with the Easter date of the Western churches; it is usually one, four or five weeks later than in the west.
On January 1, 1912, after the fall of the German Empire, the Republic of China also adopted the Gregorian calendar, which, however, was unable to assert itself due to the rule of large parts of the country by warlords . The government set up by the Kuomintang finally ordered its use from January 1, 1929 in the areas under its control. The People's Republic of China has been using it since its proclamation on October 1, 1949.
The situation in Turkey is more complicated . This adopted the Gregorian calendar from January 1, 1926 as the "International Calendar" after the previous resolution of its National Assembly on December 26, 1925. In fact, only the year after the birth of Christ was introduced and the Islamic calendar was finally abolished. In Turkey and the Ottoman Empire, the Rumi calendar had applied alongside the Islamic calendar . The Rumi calendar was originally a Julian calendar with year counting according to the Hejra, which was originally only intended for fiscal purposes, but since the 19th century after its general official introduction it had become more and more popular and increasingly supplanted the Islamic calendar. The Rumi calendar was adapted to the Gregorian calendar as early as 1917 (with the exception of the year count). Today the Gregorian calendar has also been introduced in most Islamic states and is ultimately more important than the Islamic calendar, which does not play a role in everyday life except for Islamic festivals. National holidays and other national memorial days , New Years Day , Labor Day , Mother's Day and other international holidays and commemorations are celebrated according to the Gregorian calendar. In bourgeois life, for example for employment, tenancy, etc., the Gregorian calendar is usually decisive, but not the Islamic calendar.
The fact that the Gregorian calendar was not introduced at the same time in the various countries has caused confusion to this day: both William Shakespeare and Miguel de Cervantes died on April 23, 1616, although Shakespeare survived Cervantes by ten days. George Washington's birthday celebrations were also held in the United States on various occasions on February 11th and 22nd, until a uniform holiday regulation was established under federal law.
In Germany, Austria, Switzerland and many other countries, the ISO 8601 standard applies to dates . It is based on the Gregorian calendar and extends its validity to the period before the calendar reform. This standard provides for a year zero and negative years, which do not exist in either the Julian or the Gregorian calendar.
With the calendar program iCal - developed by the computer manufacturer Apple - the jump from the Julian to the Gregorian calendar (4th to 15th October 1582) was taken into account. The Unix standard tool cal also takes this jump into account, but sets it to September 1752, corresponding to the time when the Gregorian calendar was introduced in Great Britain.
Calendar and tropical year
The calendar year is based on the tropical year (solar year) in the old definition - the period between two successive beginnings of spring (spring equals day and night), which in relation to the year 2000 is on average 365.242375 mean sunny days. Thus, the Gregorian calendar year is still a bit too long compared to the astronomical reality, namely 0.000125 days (= 11 seconds ). With this difference, the beginning of spring would only occur a whole calendar day earlier after around 8,000 years. No calendar correction would be necessary beforehand. The difference value decreases until the end of the third millennium, then approaches the value of the year 2000, which it will have reached at the beginning of the fifth millennium. From the fifth millennium onwards, the difference value increases continuously.
For more detailed information, see tropical year .
Leap year circle
The distribution scheme between common and leap years only repeats itself every 400 years. The four year long Julian leap year circle has received a hundredfold period.
The Julian switching rule is relativized in the Gregorian calendar with the help of two further rules:
- The years of the Christian calendar, which are divisible by four whole numbers, are, as before in the Julian calendar, leap years. Since every fourth calendar year is one day longer than the 365-day common years in between, the mean length of a calendar year is 365.25 days, which is too long compared to the tropical year with 365.24219 days (one day deviation after 128 years ).
- In the years (1600, 1700, 1800, 1900, 2000, 2100 etc.) that can be divided by 100 whole numbers (and thus also by four whole numbers), the leap day is omitted, contrary to the first (Julian) rule, so that the mean calendar year is 365, 24 days only deviates by about 0.0022 days from the tropical year, is a little too short (one day deviation after 457 years).
- The whole number divisible by 400 (and thus also by 100 whole number divisible) years (1600, 2000, etc.) are, contrary to the second rule and in accordance with the first rule, leap years. The mean length of the calendar year is thus 365.2425 days. The remaining difference of 0.00031 days to the mean tropical year was accepted by the reformers as negligibly small. The deviation will only be one day after about 3225 years.
Assignment between calendar dates and days of the week (solar circles)
The solar circle period is 28 years long in the Julian calendar. After this time, the assignment of the calendar data to the days of the week is repeated. In the Gregorian calendar this period is also longer, but is “only” 400 years, because this period consists of an exact whole number of weeks.
Within a century , the 28-year period also applies in the Gregorian calendar. Because the year 2000 was a leap year, it is valid for two centuries. For people living today, the distribution of their birthdays over the days of the week is repeated every 28 years until 2100.
- for other circles in the Gregorian calendar see the Easter cycle
Calendar weeks per year
The solar circle period contains exactly 20,871 weeks (wo). In each period there are 71 years with a 53rd calendar week .
- 400 a 365.2425 d / a = 146.097 d
- 146,097 days / 7 days / where = 20,871 where (400 years include 20,871 weeks)
- 400 a 52 wo / a = 20,800 wo (400 years of 52 weeks would result in 20,800 weeks)
- 20,871 where - 20,800 where = 71 where
The remaining 71 weeks are spread over 71 years with a 53rd calendar week.
Average month length and average number of weeks per month
One month (mo) long on average 30.436875 days or 4.348125 weeks, that is four weeks, two days, ten hours, 29 minutes and six seconds, or 2,629,746 seconds (ignoring leap seconds ). (A whole leap year circle of 400 a is considered.)
- 146.097 d ÷ 4.800 mo = 30.436875 d / mo
- 20,871 wo ÷ 4,800 mo = 4,348125 wo / mo
Friday the 13th.
A certain date (day and month or just day) does not fall equally often on all days of the week. The 13th of any month falls slightly more often (688 times in 400 years) on a Friday than on other days of the week (Thursday and Saturday: 684 times, Monday and Tuesday: 685 times, Sunday and Wednesday: 687 times).
In the Gregorian calendar, the Easter cycle lasts 5,700,000 years.
- List of calendar systems
- Conversion between the Julian and Gregorian calendar
- Conversion between the Julian date and the Gregorian calendar
- Calendar generator Print out the annual calendar
- Endless calendar
- For calendar invoices in general, see the web links in the Calendar article
Special Aspects of the Gregorian Calendar System
- Pope Gregory XIII: Inter Gravissimas. ( Memento of December 11, 2012 in the web archive archive.today ) Papal bull decreing the Gregorian calendar
- Alternatives to the Gregorian Calendar (private website)
- Portable library for international date and time calculations
- Hermann Grotefend: Time calculation of the German Middle Ages and the modern age
- Dates of the calendar change to wiki-de.genealogy.net; accessed on August 16, 2018
- J. Meeus: Astronomical Tables of the Sun, Moon and Planets . Willmann-Bell, Richmond 1995, ISBN 0-943396-45-X , p. 140: March equinox on March 10, 1582, 23:57:54 terrestrial time , i.e. in the area of today's Central European time zone and east of it on March 11. March.
- G. V. Coyne, M. A. Hoskin, O. Pedersen: Gregorian Reform of the Calendar: Proceedings of the Vatican conference to commemorate its 400th anniversary. Specola Vaticana, citta del vaticano 1983, p. 68, ( harvard.edu )
- JD North: The Western Calendar - "Intolerabilis, Horribilis, et Derisibilis"; Four Centuries of Discontent. In: G. V. Coyne, M. A. Hoskin, O. Pedersen: Gregorian Reform of the Calendar: Proceedings of the Vatican conference to commemorate its 400th anniversary. Specola Vaticana, citta del vaticano 1983, p. 78, ( harvard.edu )
- A. Ziggelaar: The Papal Bull of 1582 Promulgating a reform of the Calendar. In: GV Coyne, MA Hoskin, O. Pedersen: Gregorian Reform of the Calendar: Proceedings of the Vatican conference to commemorate its 400th anniversary. Specola Vaticana, citta del vaticano 1983, p. 201, harvard.edu
- Heinz Zemanek: Calendar and Chronology. Munich 1990, p. 29.
- A. Ziggelaar: The Papal Bull of 1582 Promulgating a reform of the Calendar. In: GV Coyne, MA Hoskin, O. Pedersen: Gregorian Reform of the Calendar: Proceedings of the Vatican conference to commemorate its 400th anniversary. Specola Vaticana, citta del vaticano 1983, p. 223, ( harvard.edu )
- Lorenzo Cattini, Legislazione toscana raccolta e illustrata , vol. 10, p. 208.
- Alexandre Dumas, Storia del governo della Toscana: sotto La casa de'Medici .
- Il calendario fiorentino .
- More veneto
- Ahasver von Brandt , tool of the historian. An introduction to the historical auxiliary sciences (= Urban-Bücher 33, ISBN 3-17-009340-1 ). Kohlhammer, Stuttgart 1986. p. 32.
- According to the Historisch-Biographisches Lexikon der Schweiz , article "Zeitrechnung", in Glarus reformed in 1798.
- John James Bond: Handy-book of Rules and Tables for Verifying Dates with the Christian Era: Giving an Account of the Chief Eras, and Systems Used by Various Nations, with Easy Methods for Determining the Corresponding Dates; with Regnal Years of English Sovereigns from the Norman Conquest to the Present Time, AD 1066 to 1874 . G. Bell, 1875 ( google.de [accessed February 27, 2020]).
- Cornelius Hasselblatt: History of Estonian Literature: From the Beginnings to the Present. Berlin 2006. S. XVII.
- Zakon o izjednacavanju novog i starog kalendara
- 'Έθνος', 1 Μαρτίου 1923, σελ. 1: "Το νέον ημερολόγιον" . Sometimes the date is given March 23, 1924, but that is the date of the adoption of the New Julian calendar in the Church of Greece .
- Calendar and time calculation: Introduction to the Chinese calendar kultur-in-asien.de, accessed on February 29, 2020
- Regina Kusch: When Prussia cut ten days from 1612. Deutschlandfunk , September 2, 2012, accessed on September 2, 2012 .
- Dicziunari Rumantsch Grischun , article Büman (Volume II, page 602 ff.), According to a statement from around 1910 that this was the case "40 years ago".
- Günther Winkler: Time and Law. Volume 100: of research from state and law. Springer, 1995, ISBN 978-3-211-82763-5 , p. 330 ( limited preview in Google book search).
- Peter Aufgebauer : "Between Astronomy and Politics. Gottfried Wilhelm Leibniz and the Improved Calendar of German Protestants. In: Niedersächsisches Jahrbuch für Landesgeschichte Vol. 81, 2009, pp. 385–404.
- Ludwig Ideler: Handbook of mathematical and technical chronology. 2nd volume. August Rücker, Berlin 1826, p. 325 ( archive.org ).
- Esin İleri: Textbook of the Turkish language . ISBN 3-87548-344-8 , p. 159; Retrieved February 15, 2011.
- Şükran Vahide: Islam in Modern Turkey . ISBN 978-3-8258-1794-7 , p. 207; Retrieved February 15, 2011.
- Heinz Zemanek : Calendar and Chronology , Munich, 1990.