[tz] Beginner's help request

Zefram zefram at fysh.org
Mon Oct 16 13:25:03 UTC 2017

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.  `Civilised' countries may
always announce changes some time ahead, but they don't actually say
how far ahead this is.  They don't guarantee it, and even if they did
claim to give a guarantee they'd probably occasionally break it, such
as with Brazil's short-notice rule change in 2002.  Even if you do get
a hard guarantee, it'll be for a limited period (two years?), and the
rest of the future beyond that is a guess.

And of course you can't ignore all the `uncivilised' places where even
next week's timezone is a guess.  There's also a theoretical possibility
of a DST rule (firmly in place well in advance) invoking an observational
version of the Islamic calendar, making a DST switch depend on actual
eyeball observation of the new moon.

So, overall, if you stop taking updates, then in most places you'll be
OK in practice for a few years, but some places you won't.  You can't
tell in advance which places will be OK without an update, and there's
nowhere that it's safe to go unupdated for years.


More information about the tz mailing list