FW: Possible error in Tonga time zone (forwarded with permission)

Olson, Arthur David (NCI) olsona at dc37a.nci.nih.gov
Tue Dec 7 18:57:57 UTC 1999

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-----Original Message-----
From:	Jesper Nørgaard [SMTP:jnorgard at Prodigy.Net.mx]
Sent:	Saturday, December 04, 1999 4:38 AM
To:	Olson, Arthur David (NCI)
Subject:	Possible error in Tonga time zone

Hi, Arthur David Olson.

I understand that you have done a lot of work on time zone rules to be used in
programs, and I have been using the files in elsie.nci.nih.gov for my own PC
program which you can download from
http://www.cimmyt.cgiar.org/timezone/timezone.exe, which is an installation
executable of 505 Kb. The program only works in 32-bit Windows, meaning Windows
95, Windows 98 and Windows NT 4.0 and upwards. The installation program can
guide you through the process. The program is free to copy and use, by the way.

I have noticed a change in Tonga which on
http://www.timezoneconverter.com/tzchist.html is referred to as:
"Tonga now observes DST from the first Saturday in October until the Third
Saturday in April.(Tonga will now be the first country to observe the year
2000.) "

However, in the australasia file I find the following current rule for Tonga:
# Tonga
Rule	Tonga	1999	max	-	Oct	Sat>=1	2:00s	1:00	S
Rule	Tonga	2000	max	-	Apr	Sun>=16	2:00s	-	-
Zone Pacific/Tongatapu	12:19:20 -	LMT	1901
			12:20	-	TOT	1941 # Tonga Time
			13:00	-	TOT	1999
			13:00	Tonga	TO%sT

In this case there is a discrepancy, since the third Sunday of April could fall
on 15.th. of April, meaning that in that single case (which first happens 15.th.
of April 2001) Tonga will leave Daylight Saving at Sunday the 22.nd. of April
2001 according to the above rule, but at Sunday the 15.th. of April 2001
according to the web page informal rule. My guess is that the rule really should
state "Sun>=15" instead of "Sun>=16". Note that both rules will work correctly
the year 2000, the problem arises first in 2001.

Who is right?

Hope you have a chance to look at my Time Zone Converter program by the way, and
tell me what you think. A notabene is that I by mistake put my rule for Tonga
leaving Daylight Saving as Last Saturday of April instead of Last Sunday. I will
correct that as quickly as time permits ...

Yours sincerely
Jesper Nørgaard Welen,

Jesper Nørgaard
Email: jnorgard at df1.telmex.net.mx	
CIMMYT  -  Centro Internacional de Mejoramiento de Maíz y Trigo
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