Old Style (O.S.) and New Style (N.S.) are terms sometimes used with dates to indicate that the calendar convention used at the time described is different from that in use at the time the document was being written. There were two calendar changes in Great Britain and its colonies, which may sometimes complicate matters: the first was to change the start of the year from Lady Day (25 March) to 1 January; the second was to discard the Julian calendar in favour of the Gregorian calendar.Closely related is the custom of dual dating, where writers gave two consecutive years to reflect differences in the starting date of the year, or to include both the Julian and Gregorian dates.
Beginning in 1582, the Gregorian calendar replaced the Julian in Roman Catholic countries. This change was implemented subsequently in Protestant and Orthodox countries, usually at much later dates. In England and Wales, Ireland, and the British colonies, the change to the start of the year and the changeover from the Julian calendar occurred in 1752 under the Calendar (New Style) Act 1750. In Scotland, the legal start of the year had already been moved to 1 January (in 1600), but Scotland otherwise continued to use the Julian calendar until 1752. Thus "New Style" can either refer to the start of year adjustment, or to the adoption of the Gregorian calendar.
In Russia, new style dates came into use in early 1918. Other countries in Eastern Orthodoxy adopted new style dating for their civil calendars but most continue to use the Julian calendar for religious use. In English-language histories of other countries (especially Russia), the Anglophone OS/NS convention is often used to identify which calendar is being used when giving a date.
When recording British history it is usual to use the dates recorded at the time of the event,with the year adjusted to start on 1 January. But the start of the Julian year was not always 1 January, and was altered at different times in different countries (see New Year's Day in the Julian calendar).
From 1155 to 1752, the civil or legal year in England began on 25 March (Lady Day)so for example the execution of Charles I was recorded at the time in parliament as happening on 30 January 1648 (Old Style). In newer English language texts this date is usually shown as "30 January 1649" (New Style). The corresponding date in the Gregorian calendar is 9 February 1649, the date by which his contemporaries in some parts of continental Europe would have recorded his execution.
The O.S./N.S. designation is particularly relevant for dates which fall between the start of the "historical year" (1 January) and the official start date, where different. This was 25 March in England, Wales and the colonies until 1752.
During the years between the first introduction of the Gregorian calendar in continental Europe and its introduction in Britain, contemporary usage in England started to change.In Britain 1 January was celebrated as the New Year festival, but the "year starting 25th March was called the Civil or Legal Year, although the phrase Old Style was more commonly used." To reduce misunderstandings about the date, it was normal in parish registers to place a new year heading after 24 March (for example "1661") and another heading at the end of the following December, "1661/62", a form of dual dating to indicate that in the following few weeks the year was 1661 Old Style but 1662 New Style. Some more modern sources, often more academic ones (e.g. the History of Parliament) also use the "1661/62" style for the period between 1 January and 25 March for years before the introduction of the Gregorian calendar in England.
Scotland had already partly made the change: its calendar year had begun on 1 January since 1600.
Through the enactment of the British Calendar (New Style) Act 1750 and of the Irish Parliament's Calendar (New Style) Act, 1750,Great Britain, Ireland and the British Empire (including much of what is now the eastern part of 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. The British tax year traditionally began on Lady Day (25 March) on the Julian calendar and this became 5 April, which was the "New Style" equivalent. 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.
The European colonies of the Americas adopted the new style calendar when their mother countries did. In what is now the continental United States, the French and Spanish possessions did so before the British colony. In Alaska, the change took place after the United States purchased Alaska from Russia. Friday, 6 October 1867 was followed by Friday, 18 October. Instead of 12 days, only 11 were skipped, and the day of the week was repeated on successive days, because at the same time the International Date Line was moved, from following Alaska's eastern border with Canada to following its new western border, now with Russia.
It is common in English language publications to use the familiar Old Style and/or New Style terms when discussing events and personalities in other countries, especially with reference to the Russian Empire and the very-early Russian Soviet. For example, in the article "The October (November) Revolution" the Encyclopædia Britannica uses the format of "25 October (7 November, New Style)" to describe the date of the start of the revolution.
When this usage is encountered, the British adoption date is not necessarily intended. The 'start of year' change and the calendar system change were not always adopted concurrently. Similarly, civil and religious adoption may not have happened at the same time (or even at all). In the case of Eastern Europe, for example, all of these assumptions would be incorrect.
Usually, the mapping of new dates onto old dates with a start of year adjustment works well with little confusion for events which happened before the introduction of the Gregorian calendar. For example, the Battle of Agincourt is well known to have been fought on 25 October 1415, which is Saint Crispin's Day. 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. Events in continental western Europe are usually reported in English language histories using the Gregorian calendar. For example, the Battle of Blenheim is always given as 13 August 1704. However confusion occurs when an event involves both. For example, William III of England arrived at Brixham in England on 5 November (Julian calendar), after setting sail from the Netherlands on 11 November (Gregorian calendar), in 1688.
The Battle of the Boyne in Ireland took place a few months later on 1 July 1690 (Julian calendar). This maps to 11 July (Gregorian calendar), conveniently close to the Julian date of the subsequent (and more decisive) Battle of Aughrim on 12 July 1691 (Julian). This latter battle was commemorated annually throughout the 18th century on 12 July,following the usual historical convention of commemorating events of that period within Great Britain and Ireland by mapping the Julian date directly onto the modern Gregorian calendar date (as happens for example with Guy Fawkes Night on 5 November). The Battle of the Boyne was commemorated with smaller parades on 1 July. However, the two events were combined in the late 18th century, and continue to be celebrated as "The Twelfth".
Because of the differences, British writers and their correspondents often employed two dates, dual dating, more or less automatically. For this reason, letters concerning diplomacy and international trade sometimes bore both Julian and Gregorian dates to prevent confusion: for example, Sir William Boswell writing to Sir John Coke from The Hague dated a letter "12/22 Dec. 1635".In his biography of Dr John Dee, The Queen's Conjurer, Benjamin Woolley surmises that because Dee fought unsuccessfully for England to embrace the 1583/84 date set for the change, "England remained outside the Gregorian system for a further 170 years, communications during that period customarily carrying two dates". In contrast, Thomas Jefferson, who lived at the time that the British Isles and colonies eventually converted to the Gregorian calendar, instructed that his tombstone bear his date of birth using the Julian calendar (notated O.S. for Old Style) and his date of death using the Gregorian calendar. At Jefferson's birth the difference was eleven days between the Julian and Gregorian calendars; thus his birthday of 2 April in the Julian calendar is 13 April in the Gregorian calendar. Similarly, George Washington is nowadays officially reported as having been born on 22 February 1732, rather than on 11 February 1731/32 (Julian calendar).
There is some evidence that the calendar change was not easily accepted. Many British people continued to celebrate their holidays "Old Style" well into the 19th century,a practice that according to the author Karen Bellenir reveals a deep emotional resistance to calendar reform.
The change arose from the realisation that the correct figure for the number of days in a year is not 365.25 (365 days 6 hours) as assumed by the Julian calendar but slightly less (c. 365.242 days): the Julian calendar has too many leap years. The consequence was that the basis for calculation of the date of Easter as decided in the 4th century had drifted from reality. The Gregorian calendar reform also dealt with the accumulated difference between these figures, between the years 325 and 1582 (1752 in the British Empire), by skipping 10 days (11 in the case of Great Britain, including its colonies and Ireland) to restore the date of the vernal equinox to approximately 21 March, the approximate date it occurred at the time of the First Council of Nicea in 325.
For a ready reckoner to assist in converting O.S. dates to N.S. and vice versa, see this table.
The Julian calendar, proposed by Julius Caesar in 708 Ab urbe condita (46 BC), was a reform of the Roman calendar. It took effect on 1 January 709 AUC (45 BC), by edict. It was designed with the aid of Greek mathematicians and Greek astronomers such as Sosigenes of Alexandria.
New Year is the time or day at which a new calendar year begins and the calendar's year count increments by one.
The Revised Julian calendar, also known as the Milanković calendar, or, less formally, new calendar, is a calendar proposed by the Serbian scientist Milutin Milanković in 1923, which effectively discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the Gregorian calendar that has come to predominate worldwide. This calendar was intended to replace the ecclesiastical calendar based on the Julian calendar hitherto in use by all of the Eastern Orthodox Church. From 1 March 1600 through 28 February 2800, the Revised Julian calendar aligns its dates with the Gregorian calendar, which was proclaimed in 1582 by Pope Gregory XIII for adoption by the Christian world. The calendar has been adopted by the Orthodox churches of Constantinople, Albania, Alexandria, Antioch, Bulgaria, Cyprus, Greece, and Romania.
The Coptic calendar, also called the Alexandrian calendar, is a liturgical calendar used by the Coptic Orthodox Church and also used by the farming populace in Egypt. This calendar is based on the ancient Egyptian calendar. To avoid the calendar creep of the latter, a reform of the ancient Egyptian calendar was introduced at the time of Ptolemy III which consisted of the intercalation of a sixth epagomenal day every fourth year. However, this reform was opposed by the Egyptian priests, and the reform was not adopted until 25 BC, when the Roman Emperor Augustus imposed the Decree upon Egypt as its official calendar. To distinguish it from the Ancient Egyptian calendar, which remained in use by some astronomers until medieval times, this reformed calendar is known as the Coptic calendar. Its years and months coincide with those of the Ethiopian calendar but have different numbers and names.
In British and Irish tradition, the quarter days were the four dates in each year on which servants were hired, school terms started, and rents were due. They fell on four religious festivals roughly three months apart and close to the two solstices and two equinoxes.
In the western liturgical year, Lady Day is the traditional name in some English-speaking countries of the Feast of the Annunciation, which is celebrated on 25 March, and commemorates the visit of the archangel Gabriel to the Virgin Mary, during which he informed her that she would be the mother of Jesus Christ, the Son of God.
New Year's Day, also simply called New Year, is observed on 1 January, the first day of the year on the modern Gregorian calendar as well as the Julian calendar.
The computus is a calculation that determines the calendar date of Easter. Because the date is based on an ecclesiastical or paschal "equinox" date rather than the actual astronomical equinox, there are differences between calculations done according to the Julian calendar and the modern Gregorian calendar. The name has been used for this procedure since the early Middle Ages, as it was considered the most important computation of the age.
The Swedish calendar or Swedish style was a calendar in use in Sweden and its possessions from 1 March 1700 until 30 February 1712. It was one day ahead of the Julian calendar and ten days behind the Gregorian calendar. Easter was calculated nominally astronomically from 1740 to 1844.
In astronomy, a Julian year 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 some centuries ago, 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.
The Ethiopian calendar or Eritrean calendar is the principal calendar used in Ethiopia and also serves as the liturgical year for Christians in Eritrea and Ethiopia belonging to the Eritrean Orthodox Tewahedo Church, Ethiopian Orthodox Tewahedo Church, Eastern Catholic Churches, the Coptic Orthodox Church of Alexandria, and P'ent'ay Churches. The Ethiopian calendar is a solar calendar which in turn derives from the Egyptian calendar, but like the Julian calendar, it adds a leap day every four years without exception, and begins the year on August 29 or August 30 in the Julian calendar. A gap of seven to eight years between the Ethiopian and Gregorian calendars results from an alternative calculation in determining the date of the Annunciation.
Asimov's Biographical Encyclopedia of Science and Technology is a history of science by Isaac Asimov, written as the biographies of over 1500 scientists. Organized chronologically, beginning with Imhotep and concluding with Stephen Hawking, each biographical entry is numbered, allowing for easy cross-referencing of one scientist with another. Nearly every biographical sketch contains links to other biographies. For example, the article about John Franklin Enders  has the sentence "Fleming's  penicillin was available thanks to the work of Florey  and Chain . . ." This allows one to quickly refer to the articles about Fleming, Florey, and Chain. It includes scientists in all fields including biologists, chemists, astronomers, physicists, mathematicians, geologist, and explorers. The alphabetical list of biographical entries starts with ABBE, Cleveland  and ends with ZWORYKIN, Vladimir Kosma 
The Calendar Act 1750 (c.23) was an Act of the Parliament of Great Britain. The Act had two parts: first, it reformed the calendar of England and the British Dominions so that the new legal year began on 1 January rather than 25 March ; and, second, Great Britain and its Dominions adopted the Gregorian calendar, as already used in most of western Europe.
Dual dating is the practice, in historical materials, to indicate some dates with what appears to be duplicate, or excessive digits, sometimes separated by a hyphen or a slash. This is also often referred to as double dating. The need for double dating arose from the transition from an older calendar to a newer one. For example, in "10/21 February 1750/51", the dual day of the month is due to the correction for excess leap years in the Julian calendar by the Gregorian calendar, and the dual year is due to some countries beginning their numbered year on 1 January while others were still using another date.
The Gregorian calendar is the calendar used in most of the world. It is named after Pope Gregory XIII, who introduced it in October 1582.
The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582. In countries that adopted the Gregorian calendar later, dates occurring in the interim are sometimes "Gregorianized" as well. For example, George Washington was born on February 11, 1731, as Great Britain and its possessions were using the Julian calendar with English years starting on March 25 until September 1752. After the switch, that day became February 22, 1732, which is the date commonly given as Washington's birthday.
The following is a list of the official regnal years of the monarchs of the Kingdom of England, Kingdom of Great Britain and United Kingdom from 1066. The regnal calendar is used in many official British government and legal documents of historical interest, notably parliamentary statutes.
This is the calendar for Old Style 1751, which began on Monday, 25 March, in England. The Old Style year 1751 ended on 31 December, unlike England's typical Old Style calendar, which ended with the following March, on 24 March. Hence, 1751 was the final year to begin on 25 March, and Old Style 1752 began on 1 January (Wednesday), in England, Wales, Ireland, or the American colonies (etc.), as a transition year to the New Style (N.S.) Gregorian calendar. However, both calendars had been in dual use in some regions, for many years.
This is the calendar for Old Style 1752, a leap year which began on 1 January, and dropped 3–13 September to transition to the Gregorian calendar. Previously, the Old Style calendar in England had begun on 25 March and ended with the following March, on 24 March. Because the Gregorian calendar did not have leap day in 1700, the original 10-day difference in calendars had expanded to an 11-day difference, and to compensate, 2 September was followed by 14 September, as skipping 11 days beyond 3 September. Year 1753, in England followed the full Gregorian calendar. Note, below, the shortened length of September.
The adoption of the Gregorian Calendar was an event in the modern history of most nations and societies, marking a change from their traditional dating system to the modern dating system that is widely used around the world today. Some countries adopted the new calendar from 1582, some did not do so before the early twentieth century, and others did so at various dates between; however a number continue to use a different civil calendar. For many the new style calendar is only used for civil purposes and the old style calendar remains used in religious contexts. Today, the Gregorian calendar is the world's most widely used civil calendar. During – and for some time after – the change between systems, it has been common to use the terms Old Style and New Style when giving dates, to indicate which calendar was used to reckon them.
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