[tz] On merging timezones - a radical proposal.

Guy Harris guy at alum.mit.edu
Fri May 24 06:39:43 UTC 2013

On May 23, 2013, at 11:13 PM, Random832 <random832 at fastmail.us> wrote:

> On 05/23/2013 06:51 PM, Guy Harris wrote:
>> I think they could turn POSIX TZ strings into rules for Windows, but they might have to pick up the Olson database in order to handle tz database TZ strings.
> CLDR contains a mapping.

...that maps some, but not all, Olson database zone names to Windows zones:


"Unmappable Olson Time Zones

There are two kinds of unmappable Olson time zones. The first category is a set of Olson time zones that do not have the same base offset in Windows repertoires. The following zones belong to this category for now.

 Olson time zone	 Base UTC offset
 Australia/Eucla	 +08:45
 Australia/Lord_Howe	 +10:30
 Etc/GMT-14		 +14:00
 Pacific/Chatham	 +12:45
 Pacific/Kiritimati	 +14:00
 Pacific/Marquesas	 -09:30
 Pacific/Norfolk	 +11:30

Because the mapping such zone to a Windows zone is harmful, these Olson time zones are excluded in the mapping data.

The second category is a set of Olson time zone that do not have any Windows time zones with equivalent (or similar) daylight saving time rule. The following zones belong to this category.

 Olson time zone	 Comment
 America/Adak		 -10:00/DST(US rule). (UTC-10:00) Hawaii is only the Windows zone using -10 hour offset, but no DST is observed.

 Etc/GMT+8		 -08:00/No DST. Windows has two zones with -8 hour offset - (UTC-08:00) Pacific Time (US & Canada) and (UTC-08:00) Baja California - but both of them observe DST.

 Etc/GMT+9		 -09:00/No DST. (UTC-09:00) Alaska is only the Windows zone using -9 hour offset, but it observes DST.

 Pacific/Easter		 -06:00/DST(Southern Hemisphere style rule). Windows has two zones with -6 hour offset and DST enabled - (UTC-06:00) Central Time (US & Canada) and (UTC-06:00) Guadalajara, Mexico City - but both of them uses Northern Hemisphere style DST rules.

 America/Miquelon	 -03:00/DST(Canada rule). (UTC-03:00) Greenland is the closest match (-3 hour offset, EU style DST rule). However, Canada rule used by Miquelon differs from EU rule used by Greenland by 2 or 3 weeks on DST start, 1 week on DST end.

Olson time zone with no DST above (such as Etc/GMT+8) could be mapped to a Windows time zone using the same base offset, because Windows allow users to turn off daylight saving time in the control panel or via Windows API. However, it still requires extra operation, so we do not include these mappings. Significant difference in daylight saving time rules will result mapped zone to return incorrect time zone offset (for both direction), we also exclude these DST incompatible zones from the mapping data."

They could, I guess, just say "if you have a Windows box, using a DHCP server that coughs up time zone information, on Easter Island, you're out of luck"... which I guess you are, regardless of whether you're using your DHCP server for time zone rules or not, if there are no Windows zone settings that support a -6 hour offset and Southern Hemisphere-style DST rules.

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