Corrections to timezone database
mark.davis at jtcsv.com
Mon Feb 7 02:48:14 UTC 2005
I should explain what I meant by "misleading".
Back to 1970, you know that you can find a tzid matching any place, and that
tzid will produce the right results for that place at all times back to
1970. Before 1970, the situation is a bit different. You are not guaranteed
that there will be a tzid that captures the precise behavior for every
place. You know what each tzid means, but you don't when a tzid should have
been broken into multiple ones to reflect differences in behavior that
existed back then.
By analogy, suppose that the cutoff point were 1990 instead of 1970. There
might be only one tzid for, say, China. If you used that tzid with a time
that happened before 1990, for someplace like Harbin, you'd get the wrong
results and not realize it.
Now the significance of this "completeness" depends on how important it is
to get the conversions correct. If I have a calendar program that
communicates a weekly recurring meeting, and communicates that to other
participants with the internal information <08:00, America/Los Angeles>,
then I can depend on the participants' programs being able to correctly show
on their calendars precisely when that is in their local time, which may
change from week to week. It is clearly extremely important to get current
times right, and near-future and near-past times. For specialized
applications, it is probably also important over longer periods; a large
database of data like FBI records might be an example. But for most general
programs it is not as important to get it all right back before 35 years
ago, so I think 1970 is a reasonable cut-off for the developers of the
database to guarantee that the tzids are complete.
----- Original Message -----
From: "Mark Davis" <mark.davis at jtcsv.com>
To: "David Keegel" <djk at cybersource.com.au>
Cc: "Funda Wang" <fundawang at gmail.com>; "Paul Eggert" <eggert at CS.UCLA.EDU>;
"Tz (tz at elsie.nci.nih.gov)" <tz at lecserver.nci.nih.gov>
Sent: Saturday, February 05, 2005 16:02
Subject: Re: Re: Corrections to timezone database
> True, and I should have mentioned that. The database is a bit misleading
> that it does provide for data before that point for tzids, even though
> logically some of them should be broken into several IDs based on
> differences before 1970. On the other hand, one can well understand
> a cut-off point in place, if only because it is difficult to get reliable
> data earlier, and it is not as important for implementations.
> ----- Original Message -----
> From: "David Keegel" <djk at cybersource.com.au>
> To: "Mark Davis" <mark.davis at jtcsv.com>
> Cc: "Funda Wang" <fundawang at gmail.com>; "Paul Eggert"
<eggert at CS.UCLA.EDU>;
> "Tz (tz at elsie.nci.nih.gov)" <tz at lecserver.nci.nih.gov>
> Sent: Saturday, February 05, 2005 15:54
> Subject: Re: Re: Corrections to timezone database
> > On Sat, Feb 05, 2005 at 02:55:32PM -0800, Mark Davis wrote:
> > >
> > > As far as we are concerned, the tzid is simply an internal tag,
> > > region of the Earth that has the same timezone behavior, all the way
> back to
> > > the point in time where timezones started to be used instead of solar
> > The Theory file says:
> > * Uniquely identify every national region where clocks have all
> > agreed since 1970.
> > So change "all the way back to the point in time where timezones
> > started to be used instead of solar time" to "since the year 1970."
> > David Keegel <djk at cybersource.com.au>
> > Cybersource P/L: Linux/Unix Systems Administration
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