I believe that ISO8601 is as good a definition as we have for the datetime 
stamp, the Gregorian calendar, and the Proleptic Gregorian calendar.

ISO8601 is explicit in the inclusion of year `0000` and its interpretation.

> years prior to 1583 are not automatically allowed by the standard. Instead 
> "values in the range [0000] through [1582] shall only be used by mutual 
> agreement of the partners in information interchange."

CF is a good example of mutual agreement between partners.

An expanded year representation [±YYYYY] is available, again by mutual 
agreement. and it must be prefixed with a + or − sign. 
By convention 1 BC is labelled +0000, 2 BC is labeled −0001.



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