On 13/01/11 00:59, Warner Losh wrote:
On 01/12/2011 15:41, Paul Sheer wrote:
The Olson time library has all historican timezone information.
You can choose a zone, a time (back to the 1800's) and deduce
the precise zone offset in minutes and seconds.
This is inclusive of wierd time zones that were based on the hour
since sunrise (or some similar wierd thing), as well as more basic
things like daylight savings that were tried one year, and revoked
the following year by that countries government.
There are 3064 rules in the Olson time library zone files and 434
time zones, as well as GPS information for each zone.
It would seem a number of maintainers do track time zone
announcements from all over the world and keep these
rules up to date. The most recent update was November 2010.
Further, people updating the zone rules usually provide references
for the rule change, citing a government announcement.
The Olson database is good for things since the 80's or 90's. Prior to
that, the further that you go back in the past, the less reliable and
complete the information gets. Since then the information has been
updated in real time. Prior to that it was based on research for
different areas and some of that data is still extant, while other data
might not be.
Don't get me wrong, or take this as me saying that the maintainers of
this database haven't done a herculean job of making it the best we
have. The problem is that it is hard to know for sure all the details
you need to for historical times the further back into the past you get.
There's even a nice disclaimer:
# This data is by no means authoritative;....
...
# I invented the abbreviations marked '*' in the following table
etc
The Wikipedia article points out the historical examples when thing
where different which the Olsen database does not handle, relating to
the rules being chosen. In essence it focuses on UNIX time-life first
and historical changes prior to that when possible. Again, this is not
to discredit the database and the efforts to maintain it, but it is an
effect of the prioritisations made in order to focus work on the primary
use, namely recent and current years.
Cheers,
Magnus
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