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.
A month after having decreed the reform, the pope with a brief of 3 April 1582 granted to Antonio Lilio, the brother of Luigi Lilio, the exclusive right to publish the calendar for a period of ten years. The Lunario Novo secondo la nuova riforma printed by Vincenzo Accolti, one of the first calendars printed in Rome after the reform, notes at the bottom that it was signed with papal authorization and by Lilio (Con licentia delli Superiori... et permissu Ant(onii) Lilij). The papal brief was later revoked, on 20 September 1582, because Antonio Lilio proved unable to keep up with the demand for copies.
On 29 September 1582, "Philip II of Spain decreed the change from the Julian to the Gregorian calendar. This affected much of Roman Catholic Europe, as Philip was at the time ruler over "Spain and Portugal as well as "much of Italy. In these territories, as well as in the "Polish–Lithuanian Commonwealth["citation needed] (ruled by "Anna Jagiellon) and in the Papal States, the new calendar was implemented 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 followed somewhat later de facto because of delay in communication.
Many "Protestant countries initially objected to adopting a Catholic innovation; some Protestants feared the new calendar was part of a plot to return them to the Catholic fold.
"Britain and the "British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752, followed "by Sweden in 1753.
Prior to 1917, Turkey used the lunar "Islamic calendar with the "Hegira era for general purposes and the Julian calendar for fiscal purposes. The start of the fiscal year was eventually fixed at 1 March and the year number was roughly equivalent to the Hegira year (see "Rumi calendar). As the solar year is longer than the lunar year this originally entailed the use of "escape years" every so often when the number of the fiscal year would jump. From 1 March 1917 the fiscal year became Gregorian, rather than Julian. On 1 January 1926 the use of the Gregorian calendar was extended to include use for general purposes and the number of the year became the same as in other countries.
|1582: Spain, Portugal, France, Poland, Italy,
Catholic Low Countries, Luxemburg, and colonies
|1610: Prussia||1700: 'Germany', Swiss Cantons, Protestant Low Countries, Norway, Denmark||1873: Japan||1912: China, Albania|
|1584: Kingdom of Bohemia||1648: Alsace||1752: Great Britain and colonies||1875: Egypt||1915: Latvia, Lithuania|
|1682: Strasbourg||1753: Sweden and Finland||1896: Korea||1916: Bulgaria|
|1918: USSR, Estonia|
|1919: Romania, Yugoslavia|
Difference between Gregorian and Julian calendar dates
|Gregorian range||Julian range||Difference|
|From 15 October 1582
to 28 February 1700
|From 5 October 1582
to 18 February 1700
|From 1 March 1700
to 28 February 1800
|From 19 February 1700
to 17 February 1800
|From 1 March 1800
to 28 February 1900
|From 18 February 1800
to 16 February 1900
|From 1 March 1900
to 28 February 2100
|From 17 February 1900
to 15 February 2100
|From 1 March 2100
to 28 February 2200
|From 16 February 2100
to 14 February 2200
Since the introduction of the Gregorian calendar, the difference between Gregorian and Julian calendar dates has increased by three days every four centuries (all date ranges are inclusive):
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).
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.
where is the secular difference and is the year using "astronomical year numbering, that is, use (year BC) − 1 for BC years. means that if the result of the division is not an integer it is rounded down to the nearest integer. Thus during the 1900s, 1900/400 = 4, while during the −500s, −500/400 = −2.
The general rule, in years which are leap years in the Julian calendar but not the Gregorian, is as follows:
Up to 28 February in the calendar you are converting from add one day less or subtract one day more than the calculated value. Remember to give February the appropriate number of days for the calendar you are converting into. When you are subtracting days to move from Julian to Gregorian be careful, when calculating the Gregorian equivalent of 29 February (Julian), to remember that 29 February is discounted. Thus if the calculated value is −4 the Gregorian equivalent of this date is 24 February.
Beginning of the year
|Country||Start numbered year
on 1 January
|Denmark||Gradual change from
13th to 16th centuries
|"Holy Roman Empire (Catholic states)||1544||1583|
|Spain, Poland, Portugal||1556||1582|
|"Holy Roman Empire (Protestant states)||1559||1700|
|"Great Britain and the "British Empire
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 to December from the Roman Republican period until the present.
During the Middle Ages, under the influence of the Catholic Church, many Western European countries moved the start of the year to one of several important Christian festivals—25 December (supposed "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 new 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 later 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 in England the start of the tax year remained at 25 March (O.S.), 5 April (N.S.) till 1800, when it moved to 6 April. Later in 1752 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 a specific year when a 1 January-year became the norm can be identified. 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.
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. The old date was derived from the Greek system: the earlier Supputatio Romana specified it relative to 1 January.
- In 1793 France abandoned the Gregorian calendar in favour of the "French Republican Calendar. This change was reverted in 1805.
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
"Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which calendar reference system is used for the date given. In Britain and its Colonies, where the "Calendar Act of 1750 altered the start of the year[b] 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
Extending the Gregorian calendar backwards to dates preceding its official introduction produces "a proleptic calendar, which should be used with some 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. For example, the "Battle of Agincourt is universally considered 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 seemingly 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.
The Gregorian calendar continued to employ the "Julian months, which have Latinate names and irregular numbers of "days:
- "January (31 days), from Latin mēnsis Iānuārius, "Month of "Janus", the "Roman god of gates, doorways, beginnings and endings
- "February (28 days in "common and 29 in "leap years), from Latin mēnsis Februārius, "Month of the "Februa", the "Roman festival of purgation and purification, "cognate with "fever, the "Etruscan "death god "Februus ("Purifier"),["citation needed] and the "PIE word for "sulfur
- "March (31 days), from Latin mēnsis Mārtius, "Month of "Mars", the Roman "war god
- "April (30 days), from Latin mēnsis Aprīlis, of uncertain meaning but usually derived from some form of the verb aperire ("to open") or the name of the goddess "Aphrodite
- "May (31 days), from Latin mēnsis Māius, "Month of "Maia", a Roman "vegetation goddess whose name is cognate with Latin magnus ("great") and English major
- "June (30 days), from Latin mēnsis Iūnius, "Month of "Juno", the Roman goddess of "marriage, "childbirth, and rule
- "July (31 days), from Latin mēnsis Iūlius, "Month of "Julius Caesar", the month of Caesar's birth, instituted in 44 BC as part of "his calendrical reforms
- "August (31 days), from Latin mēnsis Augustus, "Month of "Augustus", instituted by Augustus in 8 BC in agreement with July and from the occurrence during the month of several important events during his rise to power
- "September (30 days), from Latin mēnsis september, "seventh month", from its position in the "Roman calendar before 153 BC
- "October (31 days), from Latin mēnsis octōber, "eighth month", from its position in the Roman calendar before 153 BC
- "November (30 days), from Latin mēnsis november, "ninth month", from its position in the Roman calendar before 153 BC
- "December (31 days), from Latin mēnsis december, "tenth month", from its position in the Roman calendar before 153 BC
Europeans sometimes attempt to remember the number of days in each month by memorizing some form of the traditional verse ""Thirty Days Hath September". It appears in Latin, Italian, and French, and belongs to a broad "oral tradition but the earliest currently attested form of the poem is the English marginalia inserted into a "calendar of saints c. 1425:
Variations appeared in "Mother Goose and continue to be taught at schools. The unhelpfulness of such involved "mnemonics has been parodied as "Thirty days hath September / But all the rest I can't remember" but it has also been called "probably the only sixteenth-century poem most ordinary citizens know by heart". A common nonverbal alternative is the "knuckle mnemonic, considering the "knuckles of one's "hands as months with 31 days and the lower spaces between them as the months with fewer days. Using two hands, one may start from either "pinkie knuckle as January and count across, omitting the space between the "index knuckles (July and August). The same procedure can be done using the knuckles of a single hand, returning from the last (July) to the first (August) and continuing through. A similar mnemonic is to move up a "piano keyboard in "semitones from an F key, taking the white keys as the longer months and the black keys as the shorter ones.
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, 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 often 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.
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 3,030 years with respect to the current value of the "mean tropical year. However, because of the "precession of the equinoxes, which is not constant, and the movement of the "perihelion (which affects the Earth's orbital speed) the error with respect to the astronomical vernal equinox is variable; using the average interval between vernal equinoxes near 2000 of 365.24237 days implies an error closer to 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.
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 in 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 in the 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.
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
- "Calendar reform
- "Conversion between Julian and Gregorian calendars
- "Doomsday rule
- Inter gravissimas in English – Wikisource
- "Julian day calculation
- "History of calendars
- "List of adoption dates of the Gregorian calendar per country
- "List of calendars
- "Old Calendarists
- "Revised Julian calendar (Milanković) – used in Eastern Orthodoxy
- Precursors of the Gregorian reform
- "Johannes de Sacrobosco, De Anni Ratione ("On reckoning the years"), c. 1235
- "Roger Bacon, "Opus Majus ("Greater Work"), c. 1267
- Introduction to Calendars. "United States Naval Observatory. Retrieved 15 January 2009.
- Calendars Archived 1 April 2004 at the "Wayback Machine. by L. E. Doggett. Section 2.
- The international standard for the representation of dates and times, "ISO 8601, uses the Gregorian calendar. Section 3.2.1.
- See Wikisource English translation of the (Latin) 1582 papal bull "Inter gravissimas.
- Moyer (1983).
- Introduction to Calendars. (15 May 2013). "United States Naval Observatory.
- Ziggelaar (1983), p. 223.
- Nineteen-Year Cycle of Dionysius. Introduction and first argumentum.
- The first known occurrence of Common Era in English dates to 1708. Years before the beginning of the era are abbreviated in English as either BC for "Before Christ", or as BCE for "Before the Common Era" Two era names occur within the bull "Inter gravissimas itself, 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. Les canons of Les textes fondateurs du calendrier grégorien (Latin) (French)
- Richards, p. 101
- Clause 3.2.1 "ISO 8601
- The cycle described applies to the solar, or civil, calendar. If the ecclesiastical lunar rules are also considered, the lunisolar Easter computus cycle repeats only after 5,700,000 years of 2,081,882,250 days in 70,499,183 lunar months, based on an assumed mean lunar month of 29 days 12 hours 44 minutes 2 seconds. (Seidelmann (1992), p. 582) [To properly function as an Easter computus, this lunisolar cycle must have the same mean year as the Gregorian solar cycle, and indeed that is exactly the case.]
- The extreme length of the Gregorian Easter computus is due to its being the product of the 19-year Metonic cycle, the thirty different possible values of the epact, and the least common multiple (10,000) of the 400-year and 2,500-year solar and lunar correction cycles. (Walker 1945, 218)
- 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
- The last major Christian region to accept the Alexandrian rules was the "Carolingian Empire (most of Western Europe) during 780–800. The last monastery in England to accept the Alexandrian rules did so in 931, and a few churches in southwest Asia beyond the eastern border of the "Byzantine Empire continued to use rules that differed slightly, causing four dates for Easter to differ every 532 years.
- Pedersen (1983), pp. 42–43.
- For example, in the Julian calendar, at Rome in 1550, the March equinox occurred at 11 Mar 6:51 AM local mean time. "Seasons calculator", Time and Date AS, 2014.
- Ari Ben-Menahem, Historical Encyclopedia of Natural and Mathematical Sciences vol. 1 (2009), p. 863.
- Ziggelaar (1983), pp. 211, 214.
- See, for example,Tabule illustrissimi principis regis alfonsii, Prague 1401 −4 (Latin). A full set of Alphonsine Tables (including tables for mean motions, conjunctions of sun and moon, equation of time, spherical astronomy, longitudes and latitudes of cities, star tables, eclipse tables).
- For an example of the information provided see Jacques Cassini, Tables astronomiques du soleil, de la lune, des planetes, des etoiles fixes, et des satellites de Jupiter et de Saturne, Paris 1740, available at  (go forward ten pages to Table III on p. 10).
- Dreyer, J L E (2014). Tycho Brahe. Cambridge. p. 52. "ISBN "978-1-108-06871-0.
He remarks that both the Alphonsine and the Prutenic Tables are several hours wrong with regard to the time of the equinoxes and solstices.
- North, J (1989). The Universal frame: historical essays in astronomy, natural philosophy and scientific method. London. p. 29. "ISBN "0-907628-95-8.
He noted on one occasion that the Alphonsine tables differed from the Prutenic by nineteen hours as to the time of the vernal equinox of 1588.
- Swerdlow (1986).
- Meeus and Savoie (1992).
- Mezzi, E., and Vizza, F., Luigi Lilio Medico Astronomo e Matematico di Cirò, Laruffa Editore, Reggio Calabria, 2010, p. 14; p. 52, citing as primary references: Biblioteca Nazionale Centrale die Firenze, Magl. 5.10.5/a, "ASV A.A., Arm. I‑XVII, 5506, f. 362r.
- Kamen, Henry (1998). Philip of Spain. Yale University Press. p. 248.
- "Pragmatica" on the Ten Days of the Year "World Digital Library, the first known South American imprint, produced in 1584 by Antonio Ricardo, of a four-page edict issued by King Philip II of Spain in 1582, decreeing the change from the Julian to the Gregorian calendar.
- A more extensive list is available at "Conversion between Julian and Gregorian calendars
- Blackburn & Holford-Strevens (1999), p. 788.
- James Evans, The history and practice of ancient astronomy (Oxford: Oxford University Press, 1998) 169. "ISBN 0-19-509539-1.
- Explanatory Supplement to The Astronomical Ephemeris and The American Ephemeris and Nautical Almanac (London: Her Majesty's Stationery Office, 1961) 417.
- Herluf Nielsen: Kronologi (2nd ed., Dansk Historisk Fællesforening, Copenhagen 1967), pp. 48–50.
- Le calendrier grégorien en France (French)
- Per decree of 16 June 1575. Hermann Grotefend, "Osteranfang" (Easter beginning), Zeitrechnung de Deutschen Mittelalters und der Neuzeit (Chronology of the German Middle Ages and modern times) (1891–1898)
- Blackburn & Holford-Strevens (1999), p. 784.
- "John James Bond, Handy-book of rules and tables for verifying dates with the Christian era Scottish decree on pp. xvii–xviii.
- Roscoe Lamont, The reform of the Julian calendar, Popular Astronomy 28 (1920) 18–32. Decree of Peter the Great is on pp. 23–24.
- "Roman Dates: Eponymous Years". Tyndalehouse.com. Retrieved 14 September 2010.
- Mike Spathaky Old Style and New Style Dates and the change to the Gregorian Calendar: A summary for genealogists
- S. I. Seleschnikow: Wieviel Monde hat ein Jahr? (Aulis-Verlag, Leipzig/Jena/Berlin 1981, p. 149), which is a German translation of С. И. Селешников: История календаря и хронология (Издательство "Наука", Moscow 1977). The relevant chapter is available online here: История календаря в России и в СССР (Calendar history in Russia and the USSR). Anno Mundi 7000 lasted from 1 March 1492 to 31 August 1492. (Russian)
- Tuesday 31 December 1661, The Diary of Samuel Pepys "I sat down to end my journell for this year, ..."
- Nørby, Toke. The Perpetual Calendar: What about England Version 29 February 2000
- "House of Commons Journal Volume 8, 9 June 1660 (Regicides)". British History Online. Retrieved 18 March 2007.
- Death warrant of Charles I web page of the UK National Archives. A demonstration of New Style meaning Julian calendar with a start of year adjustment.
- Nørby, Toke. The Perpetual Calendar
- Benjamin Woolley, The Queen's Conjurer: The science and magic of Dr. John Dee, adviser to Queen Elizabeth I (New York: Henry Holt, 2001) p. 173
- Spathaky, Mike Old Style New Style dates and the change to the Gregorian calendar. "increasingly parish registers, in addition to a new year heading after 24th March showing, for example '1733', had another heading at the end of the following December indicating '1733/4'. This showed where the New Style 1734 started even though the Old Style 1733 continued until 24th March. ... We as historians have no excuse for creating ambiguity and must keep to the notation described above in one of its forms. It is no good writing simply 20th January 1745, for a reader is left wondering whether we have used the Old or the New Style reckoning. The date should either be written 20th January 1745 OS (if indeed it was Old Style) or as 20th January 1745/6. The hyphen (1745-6) is best avoided as it can be interpreted as indicating a period of time."
- The October (November) Revolution Britannica encyclopaedia, A demonstration of New Style meaning the Gregorian calendar.
- Stockton, J.R. Date Miscellany I: The Old and New Styles "The terms 'Old Style' and 'New Style' are now commonly used for both the 'Start of Year' and 'Leap Year' [(Gregorian calendar)] changes (England & Wales: both in 1752; Scotland: 1600, 1752). I believe that, properly and historically, the 'Styles' really refer only to the 'Start of Year' change (from March 25th to January 1st); and that the 'Leap Year' change should be described as the change from Julian to Gregorian."
- "January, n.", Oxford English Dictionary, Oxford: Oxford University Press.
- "February, n.", Oxford English Dictionary.
- Liberman, Anatoly (7 March 2007), "On a Self-Congratulatory Note", Oxford Etymologist Archives, Oxford: Oxford University Press.
- "March, n.", Oxford English Dictionary.
- "April, n.", Oxford English Dictionary.
- It's not unusual for month names to be based on natural descriptions but this etymology is sometimes doubted since no other Roman months have such names.
- "Plutarch, Life of Numa, Ch. xix.
- Scullard, Festivals and Ceremonies of the Roman Republic, p. 96.
- Forsythe, Time in Roman Religion, p. 10.
- This derivation was apparently a popular one in ancient Rome, given by "Plutarch but rejected by "Varro and "Cincius.["where?]
- "May, n.", Oxford English Dictionary.
- "June, n.", Oxford English Dictionary.
- "July, n.", Oxford English Dictionary.
- "August, n.", Oxford English Dictionary.
- "September, n.", Oxford English Dictionary.
- "October, n.", Oxford English Dictionary.
- "November, n.", Oxford English Dictionary.
- "December, n.", Oxford English Dictionary.
- Ballew, Pat (1 September 2015), "On This Day in Math", Pat's Blog.
- Anianus, Computus Metricus Manualis, Strasbourg. (Latin)
- Onofri, Francesca Romana; et al. (2012), Italian for Dummies, Berlitz, pp. 101–2.
- Bond, Otto Ferdinand; et al. (1918), Military Manual of Elementary French, "Austin: E.L. Steck, p. 11.
- Misstear, Rachael (16 January 2012), "Welsh Author Digs Deep to Find Medieval Origins of Thirty Days Hath Verse", Wales Online, Media Wales.
- Bryan, Roger (30 October 2011), "The Oldest Rhyme in the Book", The Times, London: Times Newspapers.
- "Memorable Mnemonics", Today, London: BBC Radio 4, 30 November 2011.
- The Cincinnati Enquirer, Cincinnati, 20 September 1924, p. 6.
- Holland, Norman N. (1992), The Critical I, New York: Columbia University Press, p. 64–5.
- Seidelmann (1992), pp. 580–581.
- Using value from Richards (2013, p. 587) for tropical year in mean solar days, the calculation is 1/(365.2425-365.24217)
- Meeus and Savoie (1992), p. 42
- John Herschel, Outlines of Astronomy, 1849, p. 629.
- Steel, Duncan (2000). Marking Time: The Epic Quest to Invent the Perfect Calendar. John Wiley & Sons. p. 185. "ISBN "0-471-29827-1.
- In Great Britain and its colonies, "Old Style" typically implies that the date is given in the Julian calendar with the year beginning on 25 March, whereas "New Style" implies use of the Gregorian calendar with years beginning 1 January.
- In Scotland the legal start of year had been moved to 1 January in 1600 (Mike Spathaky. Old Style New Style dates and the change to the Gregorian calendar).
- 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).
- Borkowski, K. M., (1991). "The tropical calendar and solar year", J. Royal Astronomical Soc. of Canada 85(3): 121–130.
- Barsoum, Ignatius A. (2003). The Scattered Pearls. Piscataway: Georgias Press.
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- Gregorian calendar on "In Our Time at the "BBC. (listen now)
- 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