Can I write dates in the Julian calendar using ISO 8601?
No. The Standard requires that the Gregorian calendar be used for all dates. Dates before the introduction of the Gregorian calendar are written using the proleptic Gregorian calendar. This is one of the few places where the proleptic Gregorian calendar is used. Thus the Julian date 12 March 826 must be written as 0826-03-16, because its equivalent date in the Gregorian calendar is 16 March.
