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Improvement
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Resolution: Unresolved
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Normal
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None
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None
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This can be useful for both several releases (http://bugs.musicbrainz.org/ticket/5576)
as well as several ancient artists for whom we don't know exact dates (http://en.wikipedia.org/wiki/Murasaki_Shikibu).
It could be there for any date field as well.
It would be called around in english 
and vers in french.
Sometimes having centuries instead of years could also be interesting.