[tz] Beginner's help request

Paul.Koning at dell.com Paul.Koning at dell.com
Mon Oct 16 14:21:32 UTC 2017

> On Oct 16, 2017, at 9:25 AM, Zefram <zefram at fysh.org> wrote:
> Daniel Ford wrote:
>> I'm not sure why (but I don't yet understand the database) the TZdb would
>> need to indulge in 'guessing' future DST dates/times.
> Future timezone rules are always guesses.  

The Theory file has a lot of explanation about this.

The reason is really very simple: DST rules are laws (or regulations) and like any other law, subject to change at any time at the whim of politicians.

Some places make up a new rule every year, while others make up rules that are "permanent".  But none of them actually are permanent, because they are always subject to revision later.  When you see a "guess" that typically means the actual rule was created by the politicians as a one-year rule so we have no idea wha they will do in the future.  The guess then reflects what we think they might do next year, based on what they did in the past.  Sometimes the guess is wrong.  As they say "past records are no guarantee of future performance".

As an engineer, you might be tempted to think that DST rules have some sort of rational basis.  But they don't, and that is why you need to know what the current rules are.  Think of it like your tax rate: that changes at random times also.


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