[whatwg] Dates BCE

David Singer singer at apple.com
Thu Jul 30 15:29:23 PDT 2009

At 18:19  -0400 30/07/09, Joshua Cranmer wrote:
>David Singer wrote:
>>Against that, one has to realize that "the label of the day before 
>>X" is well-defined for the day before the introduction of the 
>>Gregorian calendar, and iteratively going back to year 1, year 0, 
>>year -1, and so on.
>In neither the Gregorian nor the Julian calendars is there a year 0, 
>as used in conventional speech (formats designed for machine 
>computation treat the issue a little differently).

Right.  I was specifically referring to Proleptic Gregorian Calendar 
in the specification ISO 8601, which does.  This makes arithmetic 
('how many years') and leap calculations ('is X a leap year') simpler.


'Mathematically, it is more convenient to include a year zero and 
represent earlier years as negative, for the specific purpose of 
facilitating the calculation of the number of years between a 
negative (BC) year and a positive (AD) year. This is the convention 
used in astronomical year numbering and in the international standard 
date system, ISO 8601. In these systems, the year 0 is a leap year.

ISO 8601:
NOTE In the proleptic Gregorian calendar, the calendar year [0000] is 
a leap year.

David Singer
Multimedia Standards, Apple Inc.
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