Adoption of the Gregorian calendar in the context of "Aloysius Lilius"

The Kingdom of Romania (Romanian: Regatul României) was a constitutional monarchy that existed from 25 March [O.S. 13 March] 1881 with the crowning of prince Karl of Hohenzollern-Sigmaringen as King Carol I (thus beginning the Romanian royal family), until 1947 with the abdication of King Michael I and the Romanian parliament's proclamation of the Romanian People's Republic.

From 1859 to 1877, Romania evolved from a personal union of two principalities: (Moldavia and Wallachia) called the Unification of Moldavia and Wallachia also known as "The Little Union" under a single prince to an autonomous principality with a Hohenzollern monarchy. The country gained its independence from the Ottoman Empire during the 1877–1878 Russo-Turkish War (known locally as the Romanian War of Independence), after which it was forced to cede the southern part of Bessarabia in exchange for Northern Dobruja. The kingdom's territory during the reign of King Carol I, between 13 (O.S.) / 25 March 1881 and 27 September (O.S.) / 10 October 1914 is sometimes referred to as the Romanian Old Kingdom, to distinguish it from "Greater Romania", which included the provinces that became part of the state after World War I (Bessarabia, Banat, Bukovina, and Transylvania).

The Coptic calendar, also called the Alexandrian calendar, is a liturgical calendar used by the farming populace in Egypt and used by the Coptic Orthodox and Coptic Catholic churches. It was used for fiscal purposes in Egypt until the adoption of the Gregorian calendar on 11 September 1875 (1st Thout 1592 AM). Like the Julian calendar (and unlike the international Gregorian calendar), the Coptic calendar has a leap year every four years invariably; it does not skip leap years three times every 400 years.

Inter gravissimas (English: "Among the most serious...") was a papal bull issued by Pope Gregory XIII on 24 February 1582. The document, written in Latin, reformed the Julian calendar. The reform has conventionally come to be regarded as a new calendar in its own right, called the Gregorian calendar, named after Pope Gregory XIII. In the realm of orthodox christendom whose liturgical cycles did not apply the Inter gravissimas, the reformed calendar is noted as the New Style. The reformed Julian Calendar is used in most countries today, relatively independent of religious orientation of its populaces.

Sigismund III Vasa (Polish: Zygmunt III Waza, Lithuanian: Žygimantas Vaza; 20 June 1566 – 30 April 1632N.S.) was King of Poland and Grand Duke of Lithuania from 1587 to 1632 and, as Sigismund, King of Sweden from 1592 to 1599. He was the first Polish sovereign from the House of Vasa. Religiously zealous, he imposed Catholicism across the vast realm, and his crusades against neighbouring states marked Poland's largest territorial expansion. As an enlightened despot, he presided over an era of prosperity and achievement, further distinguished by the transfer of the country's capital from Kraków to Warsaw.

Sigismund was the son of King John III of Sweden and his first wife, Catherine Jagiellon, daughter of King Sigismund I of Poland. Elected monarch of the Polish–Lithuanian Commonwealth in 1587, he sought to unify Poland and Sweden under one Catholic kingdom, and when he succeeded his deceased father in 1592 the Polish–Swedish union was created. Opposition in Protestant Sweden caused a war against Sigismund headed by Sigismund's uncle Charles IX, who deposed him in 1599.

The Declaration of the Rights of the Peoples of Russia (Russian: Деклара́ция прав наро́дов Росси́и, romanized: Deklaratsiya prav narodov Rossii) was a document promulgated by the Bolshevik government of Russia on 15 November 1917 (2 November in Julian calendar) and signed by Vladimir Lenin and Joseph Stalin.

In astronomy, a Julian year (symbol: a or a_j) is a unit of measurement of time defined as exactly 365.25 days of 86400 SI seconds each. The length of the Julian year is the average length of the year in the Julian calendar that was used in Western societies until the adoption of the Gregorian Calendar, and from which the unit is named. Nevertheless, because astronomical Julian years are measuring duration rather than designating dates, this Julian year does not correspond to years in the Julian calendar or any other calendar, nor does it correspond to the many other ways of defining a year.

Dual dating is the practice, in historical materials, of indicating a date with what appear to be duplicate or excessive digits: these may be separated by a hyphen or a slash, or placed one above the other. The need for dual dating arose from the transition from an older calendar to a newer one. Another method used is to give the date of an event according to one calendar, followed in parentheses by the date of the same event in the other calendar, appending an indicator to each to specify which reference calendar applies.

As an example, in the date "10/21 February 1750/51" – a style seen in the records of Great Britain and its possessions – the notation arises from the prospective or previous adoption of the Gregorian calendar and a concurrent calendar reform. (The dual day number is due to the eleven days difference (at the time) between the Julian calendar date and the Gregorian one; the dual year is due to a change of start of year, from 25 March to 1 January.)

The Treaty of Roskilde was negotiated at Høje Taastrup Church and was concluded on 26 February (OS) or 8 March 1658 (NS) during the Second Northern War between Frederick III of Denmark–Norway and Karl X Gustav of Sweden in the Danish city of Roskilde. After a devastating defeat, Denmark–Norway was forced to give up a third of its territory to save the rest, the ceded lands comprising Blekinge, Bornholm, Bohuslän, Scania and Trøndelag, as well as Halland.

After the treaty entered into force, Swedish forces continued to campaign in the remainder of Denmark–Norway, but had to withdraw from the Danish isles and Trøndelag in the face of a Dano–Norwegian and Dutch alliance. The Treaty of Copenhagen restored Bornholm to Denmark and Trøndelag to Norway in 1660, while the other provinces transferred in Roskilde remained Swedish.

Niels Steensen (Danish: Niels Steensen; Latinized to Nicolas Steno or Nicolaus Stenonius; 1 January 1638 – 25 November 1686 [NS: 11 January 1638 – 5 December 1686]) was a Danish scientist, a pioneer in both anatomy and geology who became a Catholic bishop in his later years. He has been beatified by the Catholic Church.

Steensen was trained in the classical texts on science; however, by 1659 he seriously questioned accepted knowledge of the natural world. Importantly he questioned explanations for tear production, the idea that fossils grew in the ground and explanations of rock formation. His investigations and his subsequent conclusions on fossils and rock formation have led scholars to consider him one of the founders of modern stratigraphy and modern geology. The importance of Steensen's foundational contributions to geology may be gauged from the fact that half of the twenty papers in a 2009 miscellany volume on The Revolution in Geology from the Renaissance to the Enlightenment focus on Steensen, the "preeminent Baroque polymath and founder of modern geologic thought".