Robert Elz kre at munnari.OZ.AU
Tue Feb 22 20:02:46 UTC 2011

    Date:        Tue, 22 Feb 2011 17:25:59 +0000
    From:        jrl <jrl at>
    Message-ID:  <E468A544C5BEF74DBF22CAAB3FD50DE810F48C92 at UNIVERZ.terraatlas.local>

  | Architecturally, my picture of a Zone (transition), as an individual
  | element/object/entity for any given period of time will have a fixed
  | beginning and ending.

You're imagining a situation where there is a "standard time" and periods
where things are different, and while for many of us that imagination is
quite a good match to reality, handling thins this way is simply not
general enough.   At times there is simply a variation, with no
intent of anything every coming back.  Take for example the message from
Steffen Thorsen just a couple of weeks ago (Feb 8), where he said ...

straen at said:
  | Many news sources report that Russia will move clocks forward for DST as
  | usual, but not back again, so they will stay permanently on DST, from 2011. 

If you think of things as occasional (or periodic) variations in the
time then you can't handle events like that (which for the purpose of
this message I will assume is going to happen, without intending in any
way to actually confirm an event about which I have no knowledge whatever.)

The transitions in the tzdata are instead the instantaneous discontinuities
of wall clock time - those events where the TV news services (etc) all
remind everyone "Don't forget to put your clocks back (or forward) before
you go to sleep tonight".  That alteration of the normal sequence of the
flow of time is the transition (a single forward or backward variation)
that the rules describe - that's also why each rule describes a one way
switch, they don't say "from X until Y the offset will be...", they say
"from X, the offset will change to ..."

This is what we need to make clear in the documentation, and if there's
anywhere that you've seen that suggests the interpretation that you have
adopted, please point that out to us so it can be corrected.   I'm saying
this because this is not an uncommon mistake to make.  It may be that there's
something in our doc that we aren't seeing that is causing this, or it
may be other circumstances.

  | From that point of view, "max" is appropriate

Ian Abbot explained why max doesn't apply - the end of the summer time rules
for Macau had nothing at all do do with Macau's return to China, summer time
ended almost 20 years before that event after all.

abbotti at said:
  | One thing that is not clear to me at the moment is how the "%s" part of a
  | zone's format is expanded before the first transition and after the final
  | transition of a rule set.  But I'm sure this would become clear if I studied
  | the zic source code a bit more! 

Yes, this has been one of the hardest parts for people to understand.
If I have it right (and here I am not sure that I do), the right wauy
to think of it is along the same lines as above - the rules specify 
transitions, the TZ abbreviation (for whatever value it has, which is
not much) starts out at whatever it was before the period during which
the rules apply, and continues that way until the fist transition event.
Then (perhaps) a different one applies - which continues until the next
transition event.   And note that transitions either occur periodically
(within some limits) as specified by the rules, or as one offs, as specified
in the zone - either way we get a transition point, where the offset,
abbreviation, and summer time flag, can (together, or individually) alter.


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