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Re: [whatwg] <time>

From: Robert J Burns <rob@robburns.com>
Date: Thu, 12 Mar 2009 14:53:44 -0500
Cc: David Singer <singer@apple.com>, Charles McCathieNevile <chaals@opera.com>, Toby A Inkster <mail@tobyinkster.co.uk>, whatwg@lists.whatwg.org, "public-html@w3.org" <public-html@w3.org>
Message-Id: <8578B7FE-DC82-4181-BECB-D66544C98FBA@robburns.com>
To: Geoffrey Sneddon <foolistbar@googlemail.com>
Hi Geoffrey,

On Mar 12, 2009, at 11:42 AM, Geoffrey Sneddon wrote:

>
> On 10 Mar 2009, at 17:03, David Singer wrote:
>
>> At 3:22  +0100 10/03/09, Charles McCathieNevile wrote:
>>> That format has some serious limitations for heavy metadata users.  
>>> In particular for those who are producing information about  
>>> historical objects, from British Parliamentary records to  
>>> histories of pre-communist Russia or China to museum collections,  
>>> the fact that it doesn't handle Julian dates is a big problem -  
>>> albeit one that could be solved relatively simply in a couple of  
>>> different ways.
>>
>> The trouble is, that opens a large can of worms.  Once we step out  
>> of the Gregorian calendar, we'll get questions about various other  
>> calendar systems (e.g. Roman ab urbe condita <http://en.wikipedia.org/wiki/Ab_urbe_condita 
>> >, Byzantine Indiction cycles <http://en.wikipedia.org/wiki/ 
>> Indiction>, and any number of other calendar systems from history  
>> and in current use).  Then, of course, are the systems with a  
>> different 'year' (e.g. lunar rather than solar).  And if we were to  
>> introduce a 'calendar system designator', we'd have to talk about  
>> how one converted/normalized.
>
> Ultimately, why is the Gregorian calendar good enough for the ISO  
> but not us? I'm sure plenty of arguments were made to the ISO before  
> ISO8601 was published, yet that still supports only the Gregorian  
> calendar, having been revised twice since it's original publication  
> in 1988. Is there really any need to go beyond what ISO 8601 supports?

Well I can only speculate on this, but it looks to me like ISO 8601  
was focussed on dates as used in calendar formats and other  
contemporary dates. In other words it is focussed on contemporary  
dates where conversion is trivial (or it is only focussed on cultures/ 
locales using the Gregorian calendar). However, since HTML is a  
document format it needs to deal with dates that are ancient, where  
conversion is more difficult and where providing a Gregorian  
equivalent is sometimes a disputed conversion.

Keep in mind that iSO 8601 on its own does not even handle dates  
before 1582 nor after 9999. With a text based markup like HTML there's  
not really a reason to limit ourselves in such a way. Also as I've  
suggested before, most calendars share the same properties as  
Gregorian that ISO 8601 encodes:

  - era based integral years
  - ordinal weeks within years (encoded with two digits)
  - ordered months encoded with two digits (though some years may have  
leap months where 13 could appear or perhaps 04B to differentiate the  
leap month from from 04)
  - ordinal dates within months (encoded with two digits, though  
sometimes with more variability than Gregorian which are always the  
same except for February in leap years)

The notable exceptions are the Julian Day and Modified Julian Day  
which are even simpler integer or decimal represented calendars.

I agree that it would be better if a standard existed that handled  
such calendar encoding for us to reference, but it looks to me that a  
variation or enhancement of ISO 8601 could support most commonly used  
calendars and older dates in the Gregorian calendar.

By properly abstracting and allowing implementations to extend their  
support to other calendars (many of which are used today), we provide  
a more robust HTML solution, while still only requiring Gregorian  
calendar support. However, keep in mind that supporting Julian dates  
is hardly that different from supporting Gregorian dates since the  
only thing separating them after 1 AD are the leap year rules for when  
February 29th occurs.

Take care,
Rob
Received on Thursday, 12 March 2009 19:54:59 UTC

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