From: Jesper Nørgaard [SMTP:jnorgard@Prodigy.Net.mx] Sent: Saturday, December 04, 1999 4:38 AM "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.) " 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. The current rules are based on the following information from Rives McDow (1999-10-06): Tonga started its Daylight Saving on Saturday morning October 2nd at 0200am. Daylight Saving ends on April 16 at 0300am which is Sunday morning.
From this, I guessed that the informal statement ``until the Third Saturday in April'' actually means ``until 0300 on the Sunday after the third Saturday in April''. This explains why the tz database currently expects a DST change on 2001-04-22 instead of 2001-04-15.
If you have more authoritative information about Tonga, please let us know.