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This wiki uses the proleptic Gregorian calendar. The Gregorian calendar is the present-day calendar which is used for civic, secular, and commercial purposes in all or nearly all the civilized world. Proleptic means that this calendar is also used for events that occurred before the Gregorian calendar was adopted in any particular place and also for dates before it was used anywhere.
The advantage of this is that when events can be expressed in the proleptic Gregorian calendar they are in their proper order and the differences in times of events can be calculated easily. The difficulty is that dates according to various local calendars must be converted. When enough is known about the local calendar used by people in the past who wrote the records we use, the conversion is a more or less simple calculation.
In reality, there is often doubt about which calendar a writer in the past was using. Moreover many old calendars were not even consistent with themselves: depending on astronomical observations of dubious accuracy or fiat. For much of the history of calendars, calendars were a year's to-do list for a small region: taxes, religious obligations, planting, and harvest. Planting time would not be the same for people in differing latitudes so there was no urgency in making their calendars the same. People who traveled or communicated widely would notice discrepancies in ritual (such a Passover being observed twice in one year albeit in different places), but most people were unaware and unconcerned by such inconsistencies.
A striking illustration of the difference between the proleptic and the not-so-proleptic Gregorian calendar occurs for Britain and her American colonies in September of 1752. The day after Wednesday, September 2 was Thursday, September 14. If you knew the law and abided by it, you dated your checks the 2nd on Wednesday and the 14th on Thursday. Wednesday was the last lawful day of the Julian calendar (in the affected jurisdictions), and Thursday was the first day of the Gregorian calendar. (Of course if you were in a place in North America where Catholicism ruled or a Catholic functionary keeping church records, you had been on the Gregorian calendar for a long time before.) In the proleptic Gregorian calendar, the day before Thursday, September 14 is Wednesday September 13, a day which never existed on the calendars of Englishmen.
Again, there is no internal problem within the proleptic Gregorian calendar. The problem, when there is one, is in knowing enough about the calendar system of source documents to convert them correctly. In fact, almost all calendar conversions are really made by counting Julian days (not to be confused with the Julian date or Julian calendar). Julian days are the actual number of days since a certain day in middle of the Fifth millennium BCE. Of course in the early part of this range there simply are no source documents which use calendars that we know enough about to make conversions anything but guesswork.
The main calendar problem for American genealogists is to determine which calendar the author of historical documents was using.
The three possibilities for dates in North America are that the date is a Gregorian date, that it is a Julian Date Old Style, or that it is a Julian date New Style.
New Style means that January 1 is the first day of the year, when the number of the year goes up by one. This is really not quite so new, as January 1 was new year's day when the Julian calendar was introduced in the in the First century BCE. But for religious reasons the new year was observed on various dates in European countries. The Old Style new year's in England was March 25. The Old Style new year's varied in European countries, but many of them were relatively quick to adopt the Gregorian calendar. For most dates on documents in North America, the difference between N.S and O.S. affects only dates between January 1 and March 25. One year must be added to O.S. dates in that period to make them N.S.
Although New Style was part of the Gregorian calendar reform (and all Gregorian dates can be assumed to be New Style), New Style was often adopted before the Gregorian calendar was adopted.
The Julian calendar runs slow (because it had too many leap days) so observable astronomical events, such as the equinoxes, were occurring at earlier calendar dates. Adoption of the Gregorian calendar required skipping a number of days to bring the calendar into line with astronomical observations and their ancient associations with particular dates. The number of days that had to be skipped was 10 when the Gregorian calendar was first adopted in the late 16th century by predominantly Roman Catholic countries. By the time Britain and its colonies adopted the Gregorian calendar in 1752 the difference was 11 days. The difference as I write this in the early 21st century is 13 days, but the Julian calendar is no longer used anywhere except for ecclesiastical purposes in Eastern Orthodox churches.
To summarize: The New Style and Old Style are about what year it is between January 1 and March 25. Julian and Gregorian calendars are about what day of the year it is.
The period in which there is much confusion and doubt coincides with most of the Colonial period in a American history. It is usually possible to determine what the official calendar was supposed to be, but it cannot be expected that personal and family papers, church records, and clerks in remote places were up to date on what the law was and chose to conform to it.
When I am not certain, I have left dates as they are in my sources to avoid muddying the waters further with a double conversion.
More or less the whole world now uses the Gregorian calendar for business and most or all secular and civic affairs. This includes places where there are differing Christian calendars and places which were never Christian. Although calendar competition has often involved matters of faith or ideology, the advantages to international commerce of everyone using the same calendar are overwhelming.
The Gregorian calendar is the one living Americans all grew up with. Most of us understand all but perhaps the finest point (which is that years ending in the digits 00 are not leap years unless the preceding digits are divisible by 4).
This wiki uses the Gregorian calendar for page titles and to express dates except in quoted matter. Conversions for some other calendars are provided. This means the calendar is proleptic (used for dates before the calendar was adopted). As a result some historic events do not appear on their traditional dates, but cross-references should be made so that those who remember the traditional dates will be directed to the right place in the Gregorian scheme.
The Chinese calendar did not, in actual use, count the cumulative number of years from some actual or traditional date in the past. Years were numbered according the reigns of various rulers. Schemes to integrate the calendar in a cumulative count vary in their result. The current practice of using Western years with Chinese dates is used here.
Although the Chinese, Hebrew, and Arab calendars are in part or in whole based on lunar cycles, which have been subject to reasonably accurate calculation for a long time, they all have had a somewhat subjective aspect in the not-too-distant past. The Arab calendar still relies on direct observation of the new crescent moon. Whether the new moon is taken to be the dark of the moon or the first crescent, these observations are subject to variations in weather and the skill of the observer. Moreover, the non-Arab calendars attempt to reconcile with the solar year by including intercalendary days or months. When the intercalendary dates were inserted at times in the past may have been discretionary. The result is that the further in the past these calendars are projected from present day calculations, the less likely the calculations are to agree with dates that people living at the time used. With no central authority and no means of rapid communication, one day might have different dates in different places although theoretically both were using the same calendar.
Following the wisdom of my operating system and PHP, I have provided French Republican calendars for the Gregorian dates between September 22, 1792 and September 22, 1806, or in other words from before the calendar was used by anyone until well after it was used. The calendar could be brought forward simply for amusement, but no one's genealogical work would be advanced by doing so.
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