On Jan 11, 2011, at 12:03 PM, Gerard Ashton wrote:

> On 1/11/2011 1:13 PM, C. M. Sperberg-McQueen wrote (in part):
>> 7 The year zero
>> Entry 110 reads in part
>>    BC has no year zero, In the BC system the year before year 1 is 1
>>    BC. Thus '-0999' means "1000 BC".
>> By "BC" and "the BC system" I think you must mean "the Gregorian
>> calendar".  The absence of a year zero applies not just to the time
>> before the era, but also after.
> The Gregorian calendar did not create the year numbering system, it adopted an existing system. I don't read Latin, so am unable to state for certain, but I don't believe the defining documents for the Gregorian calendar requires the use of the year numbering system devised by Dionysius Exiguus, together with the system for naming years before AD 1 which was devised by Bede.

Indeed, the Gregorian calendar did not create the numbering
of years; that's one of the reasons I did not say that it did.  
But the date formats defined by ISO 8601, by XSD, and by
the EDTF document all are described as denoting dates in the
Gregorian calendar.  So what is relevant here is not when the
epoch was calculated, or by whom, but how users of the Gregorian 
calendar number the years before its epoch.

Certainly the rules promulgated by Pope Gregory for calculating
leap years could be used with some other numbering of years, but
it would be eccentric, at the least, to refer to the result as
"the Gregorian calendar".  In normal usage, that term refers to
the system of date references introduced by the Gregorian reform
of 1582 and in common use today.

ISO 8601's description of its usage of the term (section,
for example, explicitly entails the use of the year numbering system 
in common use.  Similarly, Dershowitz and Reingold, in their
book Calendrical calculations (Cambridge: CUP, 1997), are clear
that by "Gregorian calendar" they mean not just the rules for
calculating leap years but also the conventional assignment of
numbers to years.  And similarly Manfred Thaller in the manual
for his historical database system Kleio, which is of some interest
for its intelligent handling of date and calendar issues.

* C. M. Sperberg-McQueen, Black Mesa Technologies LLC