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. -zefram