[tz] Beginner's help request

Daniel Ford dfnojunk at gmail.com
Mon Oct 16 12:32:37 UTC 2017


Thanks for your observations.

"Then you lose." - Not quite!  The user can still enter their local DST
rules manually, if/when they change.

"DST rules change from year to year" - In any given year, there are bound to
be changes *somewhere*.  But for a given fixed locality, the DST rules (in
most civilised countries) tend to be pretty stable.  E.g. AU/NSW, Vic, Tas
have been the same since about 2008.

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.  In most civilised
jurisdictions these are legislated and published by governments, and as
already mentioned, are usually pretty stable.

But I'm still learning! :-)


-----Original Message-----
From: Zefram [mailto:zefram at fysh.org] 
Sent: Monday, October 16, 2017 8:59 PM
To: Daniel Ford
Cc: 'Time Zone Mailing List'
Subject: Re: [tz] Beginner's help request

Daniel Ford wrote:
>This would be fine as long as my clock has access to the TZdb each year,
>but what if the database someday ceases to exist (or be maintained)?

Then you lose.  DST rules change from year to year; even if you're only
interested in current times you need frequent updates.  In practice the
extent of the need varies between locations, but even in the most stable
cases after a few years you'd just be guessing.

The database does support this kind of guessing, however.  There's a
nearly-POSIX-TZ field in the tzfiles, which represents the current best
guess of what behaviour the zone will have after all the explicitly listed
transitions.  You can extract that field and process it indefinitely on
the small device.


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