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Understanding Genealogical Dates

If you are comparing two records and can't understand why the date of birth (for example) is slightly different when all the other information indicates it is the same person, the reason may be that the different records used Julian and Gregorian dates.

For example : 27 April 1332 (Julian calendar) is 5 May 1332 (Gregorian calendar).

Note also that the conversion has a margin of error of one day 50% of the time (e.g. half of the date conversions will be out by one day). So the date above could be 4 May or 6 May.

Date Converter (created by Ian MacInnes, Albion College)
You can compare dates by going to the following page -> Link here
Note that he refers to New style and Old style dates, with Gregorian being the New style

The following comment is courtesy of N. B. Macdonald

The historical calendar changes are fascinating. A couple of countries to watch out for are Scotland and Sweden. It is easy to assume that Scotland did as England did, but Scotland switched to New Years Day = first of January 152 years before England, in 1600, while they did change from the Julian to the Gregorian calendar at the same time as England did. Sweden wavered and twice did not implement changes as decided. Once they changed their minds and once they apparently forgot. In 1712 Sweden had 2 leap days.

The British parliament eliminated the dates 03-13 Sep 1752. In Denmark/Norway: 18 Feb 1700 was followed by 01 Mar 1700.

One has to remember that sections of North America were using different systems at the same time. Some followed Great Britain, some France and some Spain. Part of Nova Scotia was Gregorian 1605-1710, Julian 1710-1752 and Gregorian since 1752. Alaska became part of the US in 1867 and made the change then, except that the churches continued to use Old Style for a while.

Japan accepted in 1873, Russia in 1917 and 1940, China in 1949.

George Washington was born on 11 February (under the old calendar), but when he was an adult, his birthday was considered to be 22 February (under the new calendar). It is perfectly valid for an ancestor to have two birth dates, both of them correct.

All this was as confusing then as it is now, so original sources can be ambiguous. However, regrettably, as the International Genealogical Index has no facility for indicating Old Style/New Style notation, it creates ambiguities which were not present in original documents.

I've picked up the above facts all over the 'net, and hope that they all are accurate. --Beth

There are plenty of documents on the internet explaining the issues surrounding these dates, with some links from the Date Converter page (above), but here is one that I found useful http://www.fourmilab.com/documents/calendar/Opens in a new window with additional date calculators.

You will also find more useful information from these links.

http://www.medievalgenealogy.org.uk/guide/chron.shtml Opens in a new window

http://www.polysyllabic.com/CalHist.html Opens in a new window

http://www.genuki.org.uk/big/easter/ Opens in a new window

The Julian Calendar

Before Gaius Julius Caesar (654--710 AUC, 100--44 BC) and the Julian Calendar named for him, the Romans counted years of 355 days ab urbe condita, from the building of the city of Rome. 1 AUC is conventionally taken to be 753 BC, but as we shall see, the correspondence between days and years was sometimes a little confused. Because the Roman year was 10 days short with respect to the solar year, the start of the year quickly got out of step with the constellations. The priesthood arbitrarily added and subtracted days and even months which helped things drifting too far, but even so, by 708 AUC (46 BC) the year was well out of phase with the seasons.

For several years since his triumph over Pompey in 705 AUC (49 BC), Julius Caesar had been in increasingly complete command of Rome, regarded as semi-divine, and Emperor in all but name. Caesar was also a gifted astronomer, and well aware of the difficulties the existing calendar caused. He decreed that 708 AUC (46 BC) should be lengthened to 445 days in order that his new calendar be inaugurated in step with the constellations on 1st. January, 709 AUC (45 BC). His new calendar had a year of 365 days plus a leap year every fourth year with 366 days. This made the average length of the year 365.25 days, a good approximation to the real length of 365.24219 days, and the Julian calendar continued almost unchanged for many centuries.

Julius Caesar's nephew and adopted heir, Gaius Octavian Augustus (691--767 AUC, 63 BC--14 AD) became first Roman Emperor in 727 AUC (27 BC) and renamed the month Quinctilis to Augustus and stole a day from February to give his month 31 days. The base date for the calendar was moved from the founding of Rome to various dates of local importance, and it was not until the 6th. century AD when the scholar Dionysius Exiguus made a study of Easter days that the Anno Domini epoch we now use came in to being.

The Gregorian Calendar

Although small, the Julian Calendar's error of 0.0078 days per year mounted up over the centuries until by the late 1500s, the error was a noticeable 12 days. So in 1582, Pope Gregory XIII (1572--1585) decreed that the day after the 4th. October 1582 would be 15th. October 1582, and henceforth century years would not be leap years unless they were also divisible by 400. England did not adopt the Gregorian calendar until 1752, when the 2nd. September (Julian) was followed by the 14th. September (Gregorian).


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