[tz] Beginner's help request

Daniel Ford dfnojunk at gmail.com
Tue Oct 17 01:58:21 UTC 2017


Thanks for your further comments.

I accept that DST 'rules' can be ephemeral, particularly in some locations.
But I think the days of 'making a DST switch depend on actual eyeball
observation of the new moon' are perhaps past, at least in most modern
Muslim nations.  These days they tend to accept the science of celestial
motion, which predicts moon (or whatever) appearances at specified locations
with great accuracy.  You only have to Google 'ramadan 2018' (without the
quotes) to convince yourself of this.  If you know of other less predictable
events that routinely cause changes to DST start/end dates, I'd be
interested to hear about them.

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.

In that unlikely event, my clock would continue using the most recent rule
until the user's local authority decided to change the rule, at which time
the user would have to manually enter the new rule into the clock.
Hopefully this won't happen within a reasonable product lifetime!

I still have a lot of reading to do about the TZdb (thanks for the 'Theory'
pointer, Paul), but when I've done that I'll almost certainly be back with a
lot more questions.  Than you all for your patience and tolerance.


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