Talk:Proleptic Gregorian calendar

AD 4 vs 45 BC
The Julian calender wasn't in use from AD 4 but 45 BC! Rich Farmbrough 08:50, 24 Jun 2005 (UTC)


 * The last paragraph could be worded a little better. But the Julian calendar that we are familiar with did not begin until AD 4. The Julian calendar that began 45 BC had a leap year every three years instead of a quadrennial leap year. In 8 BC, Caesar Augustus removed the excessive leap years by not allowing any more leap years until AD 4, which was the first leap year of the medieval Julian calendar. See Julian calendar for all the details, especially leap years error. &mdash; Joe Kress 02:57, 25 Jun 2005 (UTC)

Astronomical vs. ordinary year notation
Astronomers usually note years before 1 A.D. by negative numbers (e.g. the year -4 etc.), not by use of "B.C.", so there's not all that much chance of confusion... AnonMoos 13:01, 24 September 2006 (UTC)


 * On the contrary, several modern historians use negative numbers for years BC, but their negative years differ by one year from the astronomer's year because these historians do not use a year zero, placing −1 immediately before 1, creating endless confusion. — Joe Kress 19:23, 25 September 2006 (UTC)

Merge
I object to the merge of most of the section Gregorian calendar into this article, especially those paragraphs, which comprise most of it, which discuss dates after 1582. Although I don't necessarily object to merging the remainder of the section discussing years before 1582, because that would leave only one small paragraph, I think any merger is useless. — Joe Kress 23:01, 18 November 2006 (UTC)

Explicitly not-proleptic
"Explicitly not-proleptic" would mean that the Roman Catholic Church explicitly stated that its new calendar could not be used for dates before its official introduction in 1582. But no such statement appears in the papal bull Inter gravissimas which promulgated it.

I have no objection to providing examples of current usage provided they are correct. Of the three examples given, only the first is correct—the last two are not. I considered simply correcting them, but instead decided to discuss them first.
 * The matryology example only gives the current practice of the Roman Catholic Church. It does not prevent those outside the Church from using the Gregorian calendar in other ways. Early Christian dates are so confused that Protestants notoriously disagree on any such date. Although most Julian dates are given the same name in the Gregorian calendar, both within and outside the Church, sometimes people convert them into the proleptic Gregorian calendar to find the nominal anniversary date.
 * The astronomical example distorts the practice of astronomers, which use neither the Julian calendar nor the Gregorian calendar when calculating dates. Instead they use the 'Julian date', which does not mean a date in the Julian calendar but instead means a day and a fraction thereof in a continuous sequence of days from a date in the remote past. The Julian date as I write this is 2454205.48333, given to a precision of about one second. Sometimes such a Julian date is used as is without any conversion, but usually it is often converted into the appropriate calendar date and time, Gregorian after 1582 and Julian before 1582. Another astronomical practice can also be confusing—the use of Julian centuries and millennia. Again, this does not mean that the Julian calendar is used, instead a Julian century means a period of 36525 days and a Julian millennia means 365250 days is applied as an offset to some 'Julian date'. Either can indicate a period before or after any date, although in modern astronomical practice the Julian epoch J2000.0 is usually used, which is the Julian date 2451545.00000, or the Gregorian calendar date noon 1 January 2000 (neither the equivalent Julian calendar date of noon 19 December 1999 nor the Julian calendar date of noon 1 January 2000 is ever used). It would be better to simply say that astronomers do not use the proleptic Gregorian calendar.
 * The Julius Caesar example is confused and a poor example unless you want to get into a long discussion of the proleptic Julian calendar. Roman sources state that Julius Caesar was assassinated on the Ides of March in the year when Julius Caesar was consul for the fifth time and Mark Antony was consul for the first time. A simple translation, not a conversion, of this is 15 March 44 BC. But during its first few decades the Julian calendar added an intercalary day every three years instead of every four years, so this date in the proleptic Julian calendar, which adds an intercalary day every four years without exception, is 14 March 44 BC. The equivalent of the nominal date in the proleptic Gregorian calendar is 13 March 44 BC, whereas the equivalent true date in the proleptic Gregorian calendar is 12 March 44 BC. In no case does the proleptic Gregorian calendar ever change the year—it is always 44 BC. In astronomical year numbering, 44 BC is written as −43 in both the Julian and Gregorian calendars because of the year 0. It is not now written as 43 BC (early French astronomers around 1700 would have said 43 BC, because they did not use negative years). A better example for you would be the coronation of Charlemagne as emperor on Chrismas Day 25 December 800, because Anno Domini years were actually used in 800 and it is well after the early confused period of the Julian calendar. Historians would not convert this using the proleptic Gregorian calendar into 29 December 800.

I have never heard of an English word tardif, and it does not appear in the Oxford English Dictionary. Nevertheless, I understand your desire to mention the use of the proleptic Gregorian calendar between 1582 and 1752 or some other year when a country adopted the Gregorian calendar. However, whether dates within that interval are converted or not is country dependent. It is normal to convert United States dates but not British dates. But that is not always the case among historians. George Washington's birth is sometimes given as 11 February both because that is the date in his family bible and because Washington himself celebrated his birthday on 11 February, even though he magnanimously accepted Happy Birthday wishes from others who insisted that it be celebrated on 22 February during his lifetime. If George Washington is given as an example then say usually. — Joe Kress 02:07, 15 April 2007 (UTC)


 * You are right. Myself, just before I stored my changes, I hesited: "explicitly" ? Of course this would mean: expressed in a statement. However the Commemoration Day decision made it clear, without any doubt. Rome, in 1582, assumed the previous error of the Julian Calendar. In the Catholic calendar, the days between October 4th and 15th don't exist. So, there is no "proleptic Gregorian Calendar" recognized by Roman Church. That's a fact.  –   Merely, the word "explicitly" is probably not-adequate.


 * "[This do] not prevent those outside the Church from using the Gregorian calendar in other ways."
 * Ha, you are right! Since Gregorian Calendar isn't "patent-protected", who wants, can manage his own changements, in various sorts of 'modified Gregorian Calendars'. This is even necessary. Since currently, G.C. is also the 'world-wide civil calendar', at least, all the Christian matryology Days don't be recognized by the civil society. Similary, Cassini modified the Christian Era by the astronomical year numbering.


 * The question is: "What's the current practice?" and "Who recognizes which calendar variant?"


 * "... both within and outside the Church, sometimes people convert them into the proleptic Gregorian calendar to find the nominal anniversary date." This, Joe, is your own, pure speculation, supported by nothing. I contest it.


 * If the astronomers (still) use the Julian Date  (and the Julian centuries, instead of the progressive von Mädler periods of 46 751 days) ,  this means that they still use the Julian calendar. However we agree: "Astronomers do not use the proleptic Gregorian calendar." Therefor, the additional reason is, that the unequal gregorian centuries are unadapted for astronomical calculations.


 * Concerning your remarks concerning the date of Caesar's death: May be a simplified example, a not-exhaustively developed one, but not a  'poor example'.
 * "In no case does the proleptic Gregorian calendar ever change the year." Since ISO8601 promotes p.G.C. and since there is also a year zero in ISO8601, it well changes the year. But we can agree that the writing AD –43 is clearer, better. Also for the rest, we don't disagree w.r.t your statements on this topic.


 * Remains the most important question: "Who really uses the so-called proleptic Gregorian calendar and ISO8601?" (exepting 'most Maya scolars')


 * I would say: No one !, and this for good reasons. :The simple truth is: "Nobody can try to implement successfully any proleptic calendar by keeping the previous Era!" This would be source of bedevilment, a vile confusion. All historians will refuse, forevermore. (Gregory XIII knew it, so he didn't try it.)
 * The imbroglio caused by the belated adoption of Gregorian Calendar by non-catholic countries is great enough. (cf. days of death of Cervantes and Shakespeare)


 * I resume. This so-called p.G.C. and its ISO8601 (not in all its parts) is inconsistent, a pseudo-newfangled cul-de-sac, a dead-end way.


 * By changing era, the proleptic rule becomes possible.
 * Then one can try to examinate whether and how "idibus Martias 710 a.u.c." is affected by the well-known Roman confusion concerning the leap years, in principle, cleared by Augustus. One can try to see if it is possible to determinate, that corresponds to the 12th, 13th day of the third month of the year... (Like this is already proposed. Caesar's death on:) CE -1835.


 * -- James175 12:01, 15 April 2007 (UTC)


 * PS. Dates between 1582–1752: the US-american historian-use is progressive, not so GB-use.* – G.W. was right. That's a good opportunity to celebrate his anniversary twice ;-) once personally, in family, the second time officially.   * Here, the best use should be the double indication: N.S. (O.S.). At least as long as one stays is AD, if not: new CE (AD O.S.).

A well known example of a Christian Julian anniversary being celebrated on its equivalent Gregorian date is the Russian Christmas, which is 25 December in the Julian calendar but is always celebrated on 7 January in the Gregorian calendar used throughout Russia. A similar example is Orthodox Easter—although it occurs between 22 March and 25 April in the Julian calendar, it is celebrated between 4 April and 8 May in the Gregorian calendar used in every Orthodox country. Another well-known example of the use of the proleptic Gregorian calendar for an anniversary is the October Revolution of the Soviet Union, which was always celebrated in the Gregorian November. Some computer languages assume that all dates are Gregorian, whether they are before 1582 or not, such as javascript. The Christian Aristeo Fernando regulary uses the proleptic Gregorian calendar for his Christan dates, even though they differ greatly from the traditional dates.

Just because both the astronomical Julian date and Julian centuries were originally named after Julius Caesar does not mean that the like-named Julian calendar is used—it is not. Astronomers want to determine the period of time that has elapsed between two astronomical events. For this purpose, Julian calendar dates are never used, nor are Gregorian calendar dates. Calendar dates are always converted into the astronomical Julian date, a count of days, before any calculation is attempted. Using actual dates would require consideration of the irregular number of days in each month between any two dates. They would never use dates like 25 March and 25 December, either Julian or Gregorian, in such a calculation. After these dates are converted into the Julian date count of days, a simple subtraction gives the elapsed period of time. This period of time can then be converted into Julian centuries by dividing it by 36525. Julian centuries in astronomy are never Julian calendar centuries, they are just a period of 36525 days without any specified beginning or end.

You misunderstand astronomical year numbering and its use in ISO 8601. 44 BC is never called "AD −43", it is simply −43. Although the year number differs, it is only another name for the same year. Indeed, AD, BC and other era designations are not allowed in ISO 8601 dates.

You cannot use a derogatory term like "abnormal" in a Wikipedia article. Your personal opinions are not allowed in Wikipedia. — Joe Kress 03:24, 17 April 2007 (UTC)

Days of the Week
Does one revise the days of the week in the Proleptic Gregorian Calendar? —Preceding unsigned comment added by 204.227.204.234 (talk • contribs)


 * No, because the Gregorian calendar did not change the days of the week when it dropped ten days in 1582. Thursday 4 October 1582 (Julian) was immediately followed by Friday 15 October 1582 (Gregorian). Thus any Julian calendar date would have the same day of the week after it is converted into the proleptic Gregorian calendar: Thursday 4 October 1582 (Julian) is Thursday 14 October 1582 (Gregorian). It is obvious that the dates have changed from 4 to 14, so if the day of the week is included, the date change can be used to determine whether a specific date was written in the Julian calendar or in the proleptic Gregorian calendar. — Joe Kress 22:10, 18 April 2007 (UTC)

Wouldn't Gregorian leap years in centuries only evenly divisible by 400 mess this up farther in the past? For example 1500 would not be a leap year in the Gregorian calendar but it would be in the Julian. In the Gregorian there would not be a February 29th but there would be in the Julian. What day of the week would February 28th be in the Proleptic gregorian calendar? —Preceding unsigned comment added by 209.193.87.154 (talk • contribs)


 * February 29, 1500 (Julian) is March 10, 1500 (Gregorian)—both are Saturdays. February 28, 1500 in the proleptic Gregorian calendar is February 19, 1500 in the Julian calendar—both are Wednesdays. All dates have one and only one equivalent in the other calendar and always have the same weekday. Although February 29, 1500 does not exist in the proleptic Gregorian calendar, it can be construed to be the day after February 28, 1500 (Gregorian), that is March 1, 1500 (Gregorian), which is equivalent to February 20, 1500 (Julian). I used Calendrica for these conversions (click on the equivalent date in any other calendar to gain access to drop-down menus of all months and days in that other calendar). — Joe Kress 22:13, 19 April 2007 (UTC)

Since the chart in the article gives the number of days that the proleptic Gregorian calendar varies frim the Julian and this varies over time with the changes not equal to a week this seemed impossible to me so I checked this out using your source, Calendrica. According to this source the days of the week ARE revised as well. For example enter a day in 100 AD and you will see that you are wrong. —Preceding unsigned comment added by 209.193.87.154 (talk • contribs)


 * Since you did not state what you did, I can only guess. In Calendrica enter any Gregorian date in AD 100, for example, September 1. In the centered box near the top appears "Wednesday, 1 September 100 (Gregorian)". In the left hand box of the next line down appears "Julian: 2 September 100 C.E.". This difference of one day is consistent with an extension of the article's table one century earlier. The first entry is a date in the proleptic Gregorian calendar and the second is the equivalent date in the Julian calendar. Only one day of the week is given because both dates are on a Wednesday. Alternately, you could click on the Julian date, making it the "entered" date. The proleptic Gregorian date still shows Wednesday. The day of the week does not change. — Joe Kress 05:10, 9 May 2007 (UTC)

Joe: I think you are mistaken about the day of the week in the proleptic Gregorian calendar. In the Julian calendar Wednesday October 3, 1582 is followed by Thursday October 14. The days in between don't exist. The proleptic Gregorian calendar has these dates. If you count back from Thursday October 14 (same in both calendars) and give each of these days a day of the week, October 3 1582 will be a Sunday in the proleptic Gregorian calendar. Since there are ten missing days added to the proleptic Gregorian calendar, and 10 modulo 7 != 0, the 3rd can't have the same day of the week in both calendars. Another example is Gregorian leap centuries before 1582. In the Julian calendar there are 29 days in February but in the proleptic Gregorian calendar, years not evenly divisible by 400 are not leap years so February has 28 days. So in the proleptic Gregorian calendar, the 28th is followed by March 1 but in the Julian calendar the 28th is followed by the 29th and then March 1. Since the length of the Julian year is different than the length of the Gregorian year, these two calendars diverge. The famous example: 13.0.0.0.0 in the Maya calendar = 9/6/-3113 (Julian) and 8/11/-3113 (Gregorian). You cite Calendrica. It agrees with me: Gregorian dates: October 13 1582 is a Wednesday and October 3 is a Sunday. Also Calendrica calculates Long Count 13.0.0.0.0 as a Wednesday in the proleptic Gregorian calendar but it's a Friday in the Julian Calendar.

If you use the same day of the week as those in the Julian calendar by rounding the Julian day number and adding one and then calculate JD modulo 7, you will have a different day of the week than the one in a historical date revised to the proleptic Gregorian calendar. Senor Cuete (talk) 14:01, 4 May 2012 (UTC)Senor Cuete


 * Gregory's papal bull specified that 4 October 1582 was to be followed by 15 October 1582. All dates, including the intervening days, 5–14 October inclusive, existed in both the Julian calendar and in the proleptic Gregorian calendar, but they identified different days, both different dates and different days of the week. A specific date like 14 October usually has different days of the week in the Julian and Gregorian calendars. The only time the days of the week will be the same is when the difference between the calendars is an integer multiple of 7, such as the centuries 200–300 (0×7), 1100–1300 (1×7), 2100–2200 (2×7), etc. Even though 14 October looks like the same date, it is not, so the calendar must be specified, such as Julian vs Gregorian or Old Style vs New Style.


 * To distinguish dates in different calendars, even when they look the same, a count of days from some remote point in the past is used. The traditional count is the Julian day number (JDN), whose day 0 was 1 January 4713 BCE Julian. This day count is the correlation of the Maya calendar, so 13.0.0.0.0 is JDN 584283 in the GMT correlation. This is a Monday in both the Julian and Gregorian calendars, but has different dates, 11 August 3114 BCE Gregorian vs 6 September 3114 BCE Julian. JDN 584283 ≡ 0 mod 7 or Monday.


 * Calendrica has some quirks. It uses negative years for the proleptic Gregorian calendar but Before the Common Era (BCE) for the Julian calendar even though the spedified year, −3113 and 3114 BCE, is the same. Long Counts are regarded as continuous, not cyclical, so the Maya Creation date 13.0.0.0.0 is 0.0.0.0.0 in Calendrica. Furthermore, a baktun less than 0 and greater than 20 is allowed. If you enter 13.0.0.0.0 into Calendrica (select Mayan Long Count in the calendar drop down menu), you are identifying the coming date at the end of this year, 21 December 2012 Gregorian or 8 December 2012 Julian. This is a Friday in both calendars because JDN 2456283 ≡ 4 mod 7 or Friday, regardless of the Western calendar, Julian or Gregorian (it is not a Wednesday). Wednesday 13 October 1582 and Sunday 3 October 1582 are only correct in the Gregorian calendar—in the Julian calendar, the corresponding dates are Saturday 13 October 1582 and Wednesday 3 October 1582. — Joe Kress (talk) 06:30, 5 May 2012 (UTC)


 * Thanks. A clear and brief explanation of this - at least a mention of the fact that days of the week are revised as well, would improve the article. Senor Cuete (talk) 15:18, 5 May 2012 (UTC)Senor Cuete


 * I guess this depends on your point of view. The classic claim is that the days of the week were not revised when the Gregorian calendar replaced the Julian calendar — the dates assigned to each day of the week did change, which is what changing the calendar does. — Joe Kress (talk) 01:53, 8 May 2012 (UTC)

Ancient dates
When ancient dates from times before the introduction of the Julian calendar are given, e.g. the foundaition of the Japanese Empire on February 11 660 B.C. or the Battle of Cannae on August 2 216 B.C. which prolepic calendar is used, the Gregorian or the Julian one? —Preceding unsigned comment added by 91.62.112.219 (talk) 12:19, 17 June 2009 (UTC)

Leap years?
It would be useful if this article had a special section that explained how leap years are defined for year zero and BC years. Most people say that a year in the Gregorian calendar is a leap year if and only if the year is "divisable by 4 but not divisable 100 except if it's divisable by 400". But 0 is kind of divisable by any integer so?? Also, in Bede's proleptic calendar there was no actual year zero so does that means year 1 BC was a leap year?

-- mnemo (talk) 11:22, 30 December 2009 (UTC)


 * After reading more carefully, I suppose it's sort of covered already... --mnemo (talk) 11:26, 30 December 2009 (UTC)

Redirect
I believe the table is not appropriate for this article, calendar conversion is a subject of it's own. If there is a desire for a good manual method of conversion, I am prepared to enter the table from the Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac (1961), under an appropriate title.

All that is left is the fact that the proleptic Gregorian calendar exists, and the leap year rules requested by Mnemo above. I think it would be best to merge that into Gregorian calendar and make this a redirect. --Jc3s5h (talk) 23:07, 19 January 2010 (UTC)

Introduction
Contains "dates preceding its official introduction in 1582". That could mislead; the Papal authority was of limited extent and effect. Remove "official" or replace it with "first", I suggest. 82.163.24.100 (talk) 20:45, 12 October 2010 (UTC)


 * I see no problem with "official" for the same reason you cite for its removal—"official" is understood to be limited by the authority promulgating it. Nevertheless, I would agree to changing the phrase to "dates preceding its introduction by the Roman Catholic Church in 1582." "First" duplicates "introduction". — Joe Kress (talk) 07:55, 14 October 2010 (UTC)

Prolepticity
There have been questions about extending the calendars before about Year Zero.

I suggest that those are most easily handled by saying that the Gregorian Calendar (ignoring Easter) repeats exactly every 400 years from year minus infinity to year plus infinity; the Julian likewise every 28 years. And that they are proleptic before their first introduction (or their introduction at the place in question?). And that the Roman Civil Calendar from BC42 to AD7 or thereabouts was an incorrect Julian Calendar.

Both can be used proleptically to minus infinity; the conversion table should not go that far, but it should in my opinion cover about three millennia centred on AD 1000. Perhaps AUC 0 should be included. Julian is in current liturgical use by the Russian Orthodox Church, so the table should go to at least AD2200. Is that postleptic?!?

The table would be easier to read with ISO 8601 dates and no words - e.g. "From 1 March 600 to 25 February 700 | From 4 March 600 to 28 February 700 | 3 days" becomes "0600-03-01 - 0700-02-25 | 0600-03-04 - 0700-02-28 | 3" and should be showable on a single line.

Elsewhere : "leap years between 45 BC and 1 BC were irregular" - no, they were regularly triennial. In English, irregular means randomish.

Except for limited year ranges, the best way to convert dates between Julian and Gregorian is to convert to/from a daycount from a past date - e.g. Chronological Julian Date starting with 0.0 being Julian BC 4713 January 1st 00:00 local time.

Algorithms are scattered in http://www.merlyn.demon.co.uk/.

82.163.24.100 (talk) 21:29, 12 October 2010 (UTC)


 * I don't know what you mean by "There have been questions about extending the calendars before about Year Zero." I do that all the time, but only as an numerical exercise. When actually dating events, historians must use the conventions of their specific area, otherwise their dates will be misunderstood. The article already discusses who uses the proleptic Gregorian calendar. The proleptic Julian calendar is not the subject of this article and should not be discussed here unless you are prepared to discuss all proleptic calendars. The proleptic Gregorian calendar can be explained by mentioning its 400-year cycle. I think minus infinity to plus infinity is too technical—just mention extending the Gregorian calendar into the past or future.


 * Prochronism is the noun corresponding to the adjective proleptic. Prochronism is dating an event too early, in this case using the Gregorian calendar before it was invented. The corresponding term for dating an event too late is metachronism, which is not really applicable to the Julian calendar because it is still in use. A slightly more extensive conversion table is at Conversion between Julian and Gregorian calendars from −500 to 2100. Hence the table here is superfluous. "Irregular" is appropriate for the Julian calendar between 45 BC and AD 4 or AD 8, because the triennial cycle was followed by no leap years at all for about 12 years. I categorically reject ISO 8601 dates because they are ugly and unnecessary, but year-month-day with a named month is OK, like 2000 January 1, which is common for astronomical dates, as is a year 0. AUC should not be included because it can have two different New Years Days, January 1 or April 21. — Joe Kress (talk) 07:55, 14 October 2010 (UTC)


 * ISO 8601 forbids the use of that standard with non-Gregorian calendars, and so does WP:MOSNUM. Jc3s5h (talk) 15:25, 14 October 2010 (UTC)


 * On second thought, the metachronistic Julian calendar, that is, the Julian calendar used after it ceased to exist, can be said to apply to Julian dating of events for which the Gregorian calendar is now used exclusively. For example, national Election Day in the United States will occur on November 2, 2010 Gregorian, but on October 20, 2010 in the metachronistic Julian calendar, 258 years after the Julian calendar ceased to exist in the United States (and its progenitor British colonies), at least for civilian dates. The metachronistic Julian calendar began for civilian dates in the same year that each country adopted the Gregorian calendar. Even though groups like the Russian Orthodox Church and the Berbers of North Africa still use the Julian calendar, no country still uses it, not even Russia, so Christmas in Russia will occur on 7 January 2011 Gregorian, which is the equivalent civilian date of 25 December 2010 in the metachronistic Julian calendar. The latter date, when regarded as a Church date, is still a Julian calendar date. — Joe Kress (talk) 05:53, 15 October 2010 (UTC)

Prolepti-plasm
Holocene calendar Conversion says:
 * ''Conversion to the Holocene Era from Julian or Gregorian AD years can be achieved by adding 10,000. BC years are converted by subtracting the BC year from 10,001.


 * A useful validity check is that the last single digits of BC and HE equivalent pairs must add up to 1 or 11.''

Is it just me, or does that really not make sense? I'd like to know if calculating the day of the week for 1 January 1 HE yields a Monday, and how to calculate where it fell in relation to mid-winter. --Pawyilee (talk)


 * Neither the day of the week nor the season is mentioned in the Holocene calendar. Both are dependent on the calendar chosen, Julian or Gregorian, both of which have been assumed. Using Calendrica, in the Julian calendar, 1 January 10000 BC (1 HE) is a Monday with a solar position of 211°, about 30° or one month after the autumnal equinox at 180°. In the Gregorian calendar, 1 January 10000 BC (−9999) is also a Monday but with a solar position of 285°, half a month after the winter solstice at 270°. Because both are a Monday, the difference between the Julian and Gregorian calendars in that year is a whole number of weeks, 77 days or 11 weeks, obtained by subtracting the two Julian days also provided.


 * Regarding the digit relationship, note that the first 10000 Holocene years are simply the corresponding BC years numbered backwards, 1–10000 HE = 10000–1 BC. The pair of least significant digits cycles through 1+0=1, 2+9=11, 3+8=11, 4+7=11, 5+6=11, 6+5=11, 7+4=11, 8+3=11, 9+2=11, 0+1=1, etc. — Joe Kress (talk) 03:50, 16 October 2010 (UTC)


 * Thanks! Please consider pasting what you wrote about Calendrica to the Holocene calendar Conversion! (

I won't do it lest it break some convention of which I'm ignorant.) --Pawyilee (talk) 13:43, 16 October 2010 (UTC)

Table of correspondence between calendars
The last entry of this table for the Proleptic Gregorian range is :

"From 11 March 1500 to 4 October 1582"

This is wrong, because 4 October 1582 was the last day of the julian calendar which corresponded to 14 October 1582 in the proleptic gregorian calendar. The last 10 days are missing. So the last line of this table should be:

Japf (talk) 19:09, 14 April 2012 (UTC)

Julian calendar extended
As there are 'a number of communities' which still use the Julian calendar - is there an equivalent term for converting Gregorian dates to the Julian ones? (Mount Athos is probably the most well known; and Tsarist-period Russia - hence the October Revolution being celebrated in November... and there was a possibly urban legend about some Russian athletes arriving at an Olympics late having used the Julian calendar) 80.254.147.68 (talk) 14:16, 11 April 2013 (UTC)

Undefined for few days
The sentence 'the number of days that the calendars differ is undefined for few days.' doesn't seem quite right. Although within the context I'm not 100% sure of the correction. SamCardioNgo (talk) 08:31, 24 October 2012 (UTC)


 * That sentence is severely messed up. Before the introduction of the Gregorian calendar there was no proleptic Gregorian calendar and certainly no difference. There aren't any undefined differences between the calendars. I don't know what the author meant but what he wrote is incomprehensible and wrong. Senor Cuete (talk) 13:43, 24 October 2012 (UTC)Senor Cuete


 * The way I read it, for a leap year, you can number the days from 1 to 365, or for a leap year, you can number the days from 1 to 366. The days are also named starting with January 1 and ending with December 31. If you are comparing a Gregorian common year to a Julian common year, or a Gregorian leap year to a Julian leap year, you can find the number that corresponds to the day name, subtract the numbers, and find the difference (you have to define a convention to determine whether the difference is considered positive or negative). But if you are comparing a Gregorian common year to a Julian leap year, it isn't obvious whether to look up the number of the day name on the list with 365 entries, or 366 entries. Since the article does not define a convention describing what to do, the difference is undefined. Jc3s5h (talk) 16:54, 24 October 2012 (UTC)


 * I'd say there are two things, the meaning, and the grammar. Surely there's a way to improve the latter at least? SamCardioNgo (talk) 11:15, 25 October 2012 (UTC)


 * No day is undefined. The way to convert to/from the proleptic Gregorian calendar is to convert to/from a Julian day number. Senor Cuete (talk) 15:08, 25 October 2012 (UTC)Senor Cuete


 * It's true that any Julian calendar date after AD 8 can be converted to a (proleptic) Gregorian calendar date, and vice versa. (In and before AD 8 there is some uncertainty about how the Romans actually implemented their calendar; it is known they didn't follow Julius Caesar's rules exactly.) One could say the difference between January 14, 2011 Gregorian and January 1, 2011 Julian is 0, because they are the same day. But some people who convert between the calendars don't want to convert to Julian Day Numbers as an intermediate stage and want to convert directly, so those people would say the difference is ± 13. Jc3s5h (talk) 16:00, 25 October 2012 (UTC)


 * True and they can do it by using the table that gives the difference between the calendars. What does this have to do with the strange incomprehensible (except to you?) text in question? Senor Cuete (talk) 16:37, 25 October 2012 (UTC)Senor Cuete


 * I don't claim the text is well-written. I'm just pointing out some related concepts; if someone want's to rewrite the text they should clearly distinguish their meaning of "difference" from other possible meanings. The table provided does not permit the conversion of certain dates, such as March 1 & 2, AD 500 Gregorian, because they are absent from the table. Jc3s5h (talk) 16:46, 25 October 2012 (UTC)


 * It's getting hard to count the colons;) But I think I might have a suggestion that we could at least discuss. My suggestion relates to one emendation (which is in square brackets), I've added the full paragraph here for context -> 'Whenever the calendars do not have corresponding days — **example given**  — the number of days that the calendars differ is undefined for [those] days.' So only word that needs to be changed is [few], which, on the original, sits where [those] now resides. Any thoughts?SamCardioNgo (talk) 18:18, 25 October 2012 (UTC)
 * The only thing that is undefined is how this confusion appeared. Jc3s5h says that March 1 & 2, AD 500 PROLEPTIC Gregorian is not defined, because they are absent from the table. My question is why? It is obvious that those days correspond to February 28 and 29, 500 in the Julian calendar. What is really undefined is the difference between calendars.To solve this problem, we only need to know the context. It seems to me that the table gives how many days we add to the Julian Calendar (that was effectively in use) to obtain a proleptic Gregorian date. If we take 27/2/500J that corresponds to 28/2/500G the difference is obviously 1. The next day is 28/2/500J and 1/3/500G. The difference is now 2 in Julian Calendar and will continue to be 2 until the year 600. You can say that for this day the difference is still 1 if we depart from the Gregorian calendar, but that is not the aim of the table - as I said above this table serves to convert julian in proleptic gregorian and not the way round. So the sentence is nonsense and the table should be completed with the missing days.Japf (talk) 19:46, 26 December 2012 (UTC)

There is no universal way to interpret a calendar conversion table. One reason for this is the word "add" does not exactly apply. Take the case of "The next day is 28/2/500J and 1/3/500G. The difference is now 2 in Julian Calendar and will continue to be 2 until the year 600." But 28 + 2 = 30. So the word "add" isn't what we want, the word we want is "convert". To convert from 28 February Julian to Gregorian, we count toward the future in some list of dates. But there are two lists to choose from, the one that includes 29 February, and the one that doesn't. If the rules for using the table don't state which list is to be used in which circumstances, the conversion is undefined. Also, only rules stated in the article count; rules proposed in the talk page do not change the undefined status unless and until they are incorporated into the article. Jc3s5h (talk) 21:33, 26 December 2012 (UTC)
 * I disagree. "Gregorian ahead" means "in comparison to the julian calendar" so there is no ambiguity. Nevertheless, you can complete the table with all the dates, and add a footnote saying that the date difference may depend in some dates if we are counting the days in the gregorian or in the julian calendar. nThe present table say that we can't stablish a correspondence between 28/2/500J and 1/3/500G, and that is wrong.Japf (talk) 02:29, 27 December 2012 (UTC)


 * I disagree with "The present table say that we can't stablish a correspondence between 28/2/500J and 1/3/500G, and that is wrong." The present table does not state which Gregorian date corresponds to 28 February 500, but it does not state that the equivalent date is unknown. The problem is that not enough rules are stated to say whether conversion between 28 February 500 Julian and 1 March 500 should be done by counting forward 2 dates on a date list that includes 29 February, or forward 1 date on a list that omits 29 February. Jc3s5h (talk) 01:15, 28 December 2012 (UTC)
 * And if we put a fotenote stating "The values shown in the column Gregorian ahead mean days added to the date in the Julian Calendar". Anything must be done because the table can't remain with undefined dates like it is now.Japf (talk) 15:48, 31 December 2012 (UTC)
 * Dates are not added, they are converted. The method of conversion must be stated unambiguously. Jc3s5h (talk) 00:31, 1 January 2013 (UTC)
 * What do you think about a footnote stanting "without counting the February 29 of the julian calendar". A compromise solution must be found. The present table is inacurrated.Japf (talk) 17:10, 1 January 2013 (UTC)
 * Yes it's a good idea (also equivalent to counting days in the Gregorian Calendar). Karl (talk) 13:11, 3 January 2013 (UTC)

ISO 8601
The proleptic Gregorian calendar is explicitly required for all dates before 1582 by ISO 8601:2004 (clause 4.3.2.1 The Gregorian calendar) if the partners to information exchange agree.


 * This doesn't make much sense to me. If all partners agree to do it that way, then doing it that way is required by the definition of "agreement". Is ISO 8601 trying to define "agree", as well as defining time?  This reads like a bit of bureaucratic gobbledygook dropped into Wiki without translating it into human language first.  If you are required to follow an agreement, but only if you first agree to the agreement, wouldn't the natural way to say that be "ISO 8601 explicitly allows partners to agree to use the proleptic calendar" or something like that?

Jeff Janes (talk) 17:53, 1 March 2016 (UTC)


 * The actual situation is that people claiming they adhere to ISO 8601 must use either the Gregorian or proleptic Gregorian calendar; other calendars, such as the Julian, are not allowed. Also, if some people who want to exchange data say they adhere to ISO 8601 and that's all they agree on, then they must not express any dates before January 1, 1583 (or October 15, 1582, the standard contradicts itself on this point). If the parties exchanging information want to make a further agreement, they can agree to use the proleptic Gregorian calendar and allow the expression of earlier dates.


 * What if some parties exchanging information agree to use the Julian calendar? Then they are not adhering ISO 8601, and if they say they are, their claim is false.


 * I agree the article is badly worded and should be improved. Jc3s5h (talk) 12:47, 2 March 2016 (UTC)

Accuracy of the table?
I noticed that, for the period from 1 March 300 to 28 February 500, there is a difference of one day between the two calendars. As this period includes the First Council of Nicaea, to which the Gregorian count of days was supposed to revert to (regarding calculations of the date of Easter), surely there shouldn't be any difference at all? Or was the calendar inaccurately adjusted? Chaotic Enby (talk) 09:06, 19 April 2023 (UTC)


 * From what I've read, there was a general goal to restore the March equinox to, on average, March 21, which the reformers believed was the average March equinox at the time of the First Council of Nicaea. I've never seen any mention of the reformers thinking about a proleptic Gregorian calendar or how it would compare to the Julian calendar. Jc3s5h (talk) 12:15, 19 April 2023 (UTC)
 * They probably didn't think of it in these terms, but if they wanted to put the Gregorian calendar back in sync with the time at the First Council of Nicaea, it would be equivalent with having the proleptic calendar sync back with the Julian calendar at that date, right? Is there something I'm not seeing, or is there actually a one-day mistake that hasn't been commented on yet? Chaotic Enby (talk) 13:48, 20 April 2023 (UTC)
 * This is discussed extensively in Gregorian Reform of the Calendar:Proceedings of the Vatican Conference to Commemorate its 400th anniversary 1582 - 1982. In particular see the chapter "The Papal Bull of 1582 Promulgating a Reform of the Calendar" by August Ziggelaar, starting on page 201 (as printed in the text, not as shown in the PDF page count). In summary, it was known that the traditional method of calculating Easter, as implemented by Dionysius Exiguus, treated March 21 as the date of the March equinox. The goal was to restore the average date of the March equinox to March 21. The correct way to judge whether the reformers made an error would be to calculate the average date of the March equinox near 1583 and see if it's March 21. Whether or not the difference between the Julian calendar and the Gregorian calendar was 0 days in 325 was not a criterion of the correctness of the reform. Jc3s5h (talk) 15:39, 20 April 2023 (UTC)

Date of Roger Bacon’s Calculations
The article mentions that Roger Bacon estimated the inaccuracies of the calendar system during his time "c. 1200", when he wasn't even born until c. 1219. Perhaps change "c. 1200" to say "in the 13th century" in a similar fashion to how Bede's contributions are mentioned? Geographynerd101 (talk) 12:42, 22 May 2023 (UTC)