Default time zone for a location (previously Europe/London)

Ken Pizzini "tz." at explicate.org
Tue Sep 5 07:15:45 UTC 2006


On Tue, Sep 05, 2006 at 07:47:23AM +0200, Srdjan Krajnalic wrote:
> Thank you for answering. That solution crossed my mind, but not being
> thoroughly familiar with data I thought better to ask

Sure...

>  because the time zone
> that appears to be appropriate is from the second rule line for the same
> year, but in same cases there is only one rule to apply to the given year.

I think we're still not explaining it right...

> So, if the only cases are "one or more lines, if two or more go with the
> second name" that's good enough :-)

The choice is not not betwen first- and second- listed: the one to choose
is whichever is the oldest entry (by "From" year) for standard time.
And by "for standard time", I mean that the "SAVE" column is zero.


> Another thing that might be worth mentioning - the problem from the previous
> letter may also occur in the middle of zone rules. For example:
> 
> Rule    Algeria 1918    only    -       Mar      9      23:00s  1:00    S
> Rule    Algeria 1919    only    -       Mar      1      23:00s  1:00    S
> Rule    Algeria 1920    only    -       Feb     14      23:00s  1:00    S
> Rule    Algeria 1920    only    -       Oct     23      23:00s  0       -
> Rule    Algeria 1921    only    -       Mar     14      23:00s  1:00    S
> Rule    Algeria 1921    only    -       Jun     21      23:00s  0       -
> Rule    Algeria 1939    only    -       Sep     11      23:00s  1:00    S
> Rule    Algeria 1939    only    -       Nov     19       1:00   0       -
> 
> In this case, between 1921-Jun-21 and 1939-Sep-11, the need again arises to
> resort to the default zone line GMTOFF. In case of Algiers, the last 1921
> SAVE is appropriate through 1939-Sep-11 (0 SAVE in GMT), but in other cases
> it would not be.

Uh... no magic --- the last transition left us there, and we have
yet to see the next transition: i.e., once the transition from
daylight saving to standard time happened in June of 1921, Algiers
continued to stay on standard time until the next transition in
September of 1939.

The key is that the Rule entries do not directly specify what
the offset is for a given date, but rather they express the set of
when transitions between standard and daylight saving time occur.
The offset from UTC (nee GMT) is determined by the algebraic sum
of the GMTOFF specified in the current Zone rule and the SAVE
value specified in the most-recently-invoked Rule entry.
The name assigned to the zone is determined by the FORMAT entry
of the current Zone rule, possibly modified by the LETTER/S
entry of the most-recently-invoked Rule entry.

Put another way, the Zone and Rule entries are intended as a more
compact and more readily edited way of expressing *transitions*
within a time zone.  Omitting the GMT/UTC offsets, this is what
the entry for Algiers might look like written out in "longhand":
 Fri 1911-03-10 23:59:59 PMT   ->        23:50:39 WET
 Wed 1916-06-14 22:59:59 WET   ->  06-15 00:00:00 WEST
 Sun 1916-10-01 23:59:59 WEST  ->        23:00:00 WET
 Sat 1917-03-24 22:59:59 WET   ->  03-25 00:00:00 WEST
 Sun 1917-10-07 23:59:59 WEST  ->        23:00:00 WET
 Sat 1918-03-09 22:59:59 WET   ->  03-10 00:00:00 WEST
 Sun 1918-10-06 23:59:59 WEST  ->        23:00:00 WET
 Sat 1919-03-01 22:59:59 WET   ->  03-02 00:00:00 WEST
 Sun 1919-10-05 23:59:59 WEST  ->        23:00:00 WET
 Sat 1920-02-14 22:59:59 WET   ->  02-15 00:00:00 WEST
 Sat 1920-10-23 23:59:59 WEST  ->        23:00:00 WET
 Mon 1921-03-14 22:59:59 WET   ->  03-15 00:00:00 WEST
 Tue 1921-06-21 23:59:59 WEST  ->        23:00:00 WET
 Mon 1939-09-11 22:59:59 WET   ->  09-12 00:00:00 WEST
 Sun 1939-11-19 00:59:59 WEST  ->        00:00:00 WET
 Sun 1940-02-25 01:59:59 WET   ->        03:00:00 CET
 Mon 1944-04-03 01:59:59 CET   ->        03:00:00 CEST
 Sun 1944-10-08 01:59:59 CEST  ->        01:00:00 CET
 Mon 1945-04-02 01:59:59 CET   ->        03:00:00 CEST
 Sun 1945-09-16 00:59:59 CEST  ->        00:00:00 CET
 Sun 1946-10-06 23:59:59 CET   ->        23:00:00 WET
 Sat 1956-01-28 23:59:59 WET   ->  01-29 01:00:00 CET
 Sat 1963-04-13 23:59:59 CET   ->        23:00:00 WET
 Sun 1971-04-25 22:59:59 WET   ->  04-26 00:00:00 WEST
 Sun 1971-09-26 23:59:59 WEST  ->        23:00:00 WET
 Thu 1977-05-05 23:59:59 WET   ->  05-06 01:00:00 WEST
 Thu 1977-10-20 23:59:59 WEST  ->  10-21 00:00:00 CET
 Fri 1978-03-24 00:59:59 CET   ->        02:00:00 CEST
 Fri 1978-09-22 02:59:59 CEST  ->        02:00:00 CET
 Thu 1979-10-25 23:59:59 CET   ->        23:00:00 WET
 Thu 1980-04-24 23:59:59 WET   ->  04-25 01:00:00 WEST
 Fri 1980-10-31 01:59:59 WEST  ->        01:00:00 WET
 Thu 1981-04-30 23:59:59 WET   ->  05-01 01:00:00 CET

HTH,
		--Ken Pizzini



More information about the tz mailing list