Someone posed the question about guidelines and standards for dates. For our archival collection we use the rules set out in the "Rules for Archival Description" or RAD - very helpful! 
(see http://www.cdncouncilarchives.ca/archdesrules.html )


25 Jan. 1982 ......... definite date
[ca. 1980]................... approximate date 
[before 1867]...........terminal date
[after 5 Jan. 1867].....terminal date
[1892 or 1893]..........one year or the other			 
1982-1984..............  date range
[199-? - 200-?].......  date range with uncertain decade
[2000?]..................................  probable date
[197-]....................................  decade certain
[186-?].................................. probable decade 
[19-]..............................century certain
[20-]..............................century certain
[18-?].............................probable century

The square brackets indicate uncertain dates. And I believe that the Circa dates are the standard 10 year range (5 years on either side of the date).

I hope that this helps.

Carol Reid 
Collections Manager, Archives 
Canadian War Museum 
Gestionnaire de collection, Archives 
Musée Canadien de la guerre 
1 Vimy Place, Ottawa, ON, Canada, K1R 1C2 
www.warmuseum.ca 

tel: (819) 776-8661 
fax: (819)776-8623 
e-mail: [log in to unmask] 

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