[tz] Beginner's help request

[Paul.Koning at dell.com]

> On Oct 16, 2017, at 9:58 PM, Daniel Ford <dfnojunk at gmail.com> wrote:
> 
> ...
> Nevertheless, I understand that DST rules *do* change (and even timezones
> can change, though more rarely), so my intention is certainly to try to
> access the TZdb (or a derivative thereof) for my clock.  But I would still
> prefer to download the current 'rule' rather than the current dates, so I
> can continue calculating the changeover events in the (unlikely) future
> absence of the TZdb.

I'm not sure why you got the impression that the TZDB gives you transition dates, rather than the rules you were very sensibly looking for.

The TZDB is exactly that: a set of rules.  The latest one for any location is usually of the form "DST ends on the first sunday of november at 2:00 am local time, until further notice".  All historic rules (no longer current) of course have a start and end date.  So an application that uses the TZDB always does what you describe: it uses the rule it has been given to calculate all future times.  The reason new TZDB releases occur perhaps a dozen times per year is precisely that the old rule has been superseded by a new political act.  If you aren't up to date, you'll be in trouble anywhere from months to many years out, depending on your location -- because some change annually (occasionally even more often) while others remain stable for years or even decades.

	paul