FW: Time zone: the next generation

Olson, Arthur David (NIH/NCI) olsona at dc37a.nci.nih.gov
Thu Mar 10 14:24:04 UTC 2005

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-----Original Message-----
From: Alois Treindl [mailto:alois at astro.ch] 
Sent: Thursday, March 10, 2005 5:54 AM
To: Olson, Arthur David (NIH/NCI)
Cc: Tz (tz at elsie.nci.nih.gov)
Subject: Re: Time zone: the next generation

On Thu, 3 Mar 2005, Olson, Arthur David (NIH/NCI) wrote:
> *	Do we handle Julian/Gregorian transitions?

Yes, but with an override feature in the API.

One of the problems is that many history texts contain converted calendar
dates. For the consistency of a particular history book, biography etc,
authors have a tendency to convert all calendar data contained in them into
one system, even if at the given date and location a different calendar
might have applied.

If a related calendar/timezone database makes strict assumptions about which
calendar applied for a given date, then new problems are introduced.

> *	Do we allow control of skipping the year zero?

Both styles of year counting have a name.
The one with year zero is called 'astronomical year counting style' and it
indicates years BC with negative year numbers.
The one without year zero is called 'historical year counting style' and it
indicates years BC with the suffix BC or BCE or something similar, but never
with a plus/minus sign.

I think the documentation should be clear, but the API needs only to support
one style.

> *	Do we handle pegging of far-past and far-future times?

Aside from calendar system changes, no time zone data are known for dates
before the 1800s. There is an issue whether true solar time or mean solar 
time was used for the recording of a particular historical event. But I do 
not see how tzdata should be able to answer this question.

E.g. when J.W.Goethe mentions in one of his works that he was born at noon
on 28 August 1749 (greg. calendar) in Frankfurt:
Does he mean true solar time (as indicated by a sun dial) or mean solar 
time (as measured by an astronomical clock in an observatory)?

tzdata cannot know this, and should not claim that it knows.
There was no clear transition between true solar time and mean solar time 
reckoning, before the formal introduction of timezone standards in the 
19th century.

Alois Treindl

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