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(Great, now I'm all paranoid about double dates and how they're affecting my data...)

Several references have been made to the "double date window", period of time where the Julian and Gregorian calendars overlapped. I can assume the start of the window was at the inauguration of the Gregorian calendar (1582), but when did it end?

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It depends on the region. –  American Luke Oct 10 '12 at 21:39
    
I think this should be in the FAQ –  M Smith Oct 10 '12 at 22:23
    
@MSmith What should? –  American Luke Oct 11 '12 at 14:36
    
@Luke This question is a very common question for people getting going in their family history research. –  M Smith Oct 12 '12 at 18:58
    
A good article on the implications of interpreting calendars before the change-over window is at geneamusings.com/2012/12/… –  Fortiter Dec 14 '12 at 6:41

2 Answers 2

up vote 6 down vote accepted

The Gregorian calendar was accepted at many different times by countries around the world, the last accepting it in the 20th century. A list of countries and dates can be found via Google. Catholic countries accepted the change immediately, since it was a papal edict; protestant and non-Christian countries were reluctant (polite way of saying violently opposed) to obeying the Roman church. They straggled along, converting when the practical advantages became great enough.

Of most importance to genealogists in the English speaking world, the United Kingdom and its colonies made the change in 1752, when the second of September was followed by the 14th of September.

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TL; DR: For most parties, the window ended in September of 1752. –  fbrereto Oct 10 '12 at 22:04
    
+1 for "polite way of saying violently opposed" –  fbrereto Oct 10 '12 at 22:36
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Short answers can be misleading. September 1852 is relevant if you are limited to British ancestors. In Russia, the Window extends beyond the Revolution of 1917. Need to emphasise where dates were recorded as well as when. –  Fortiter Oct 11 '12 at 4:11
    
@Fortiter: Good point. Does a resource exist with relevant countries and their crossover dates? –  fbrereto Oct 11 '12 at 18:28

The need for dual date representations varied from nation to nation depending upon when they made the transition from a Julian to a Gregorian calendar (and, to a lesser extent, when their trading partners made the transition).

The most comprehensive list I am aware of is posted at http://www.webexhibits.org/calendars/year-countries.html. I cannot vouch for the accuracy of all these claims but it does provide a starting point for checking on particular countries.

Remember also to take account of the impact in their colonies of decisions made in London, Paris or Madrid.

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I am the editor of a genealogy society newsletter and would like to use the information from the date the countries switch calenders in the website you mentioned. I looked to see if there was a contact person to request permission to use this info (of course the article and web address would be listed as the source for the article in the newsletter) but could not find any way to request use of this article in our newsletter. Any suggestions on how to get permission for use of this chart? –  S Hamby Oct 23 '12 at 16:47
    
Welcome to the site, @SHamby. FYI, "Countries' Calendar Reform" appears part of www.webexhibits.org. The about section of the latter website, section titled "Links & Copies," reads, "Please feel free to link to any pages on this site, or reproduce excerpts or screen grabs for editorial purposes. You can copy, distribute, or display many of the exhibits, unless otherwise noted. To use exhibits for commercial purposes, please contact us." See webexhibits.org/about/about.html –  GeneJ Oct 23 '12 at 17:02

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