[tz] Fwd: Re: [english 83%] Re: historic Oslo times

[Alan Barrett]

On Tue, 10 Mar 2015, Meno Hochschild wrote:
> Okay, now I have done my own investigations and found following 
> answer:
>
> If the switch (expressed as UNTIL-time in a ZONE-line) is before 
> any dst-rule valid for given UNTIL-related year then we have to 
> apply the previous daylight saving offset valid at that time 
> (previous means before the switch).

That sounds like one of the consequences of the way Zones and 
Rules interact.

My explanation would be more like this:

The lines associated with a "Rule" define the instants of 
transition between standard time and DST, and the offset 
associated with each transition.  These transitions are uses by 
all Zones that follow the Rule.

A line associated with a "Zone" may refer to a particular Rule, 
along with the instant at which the zone starts following the 
Rule, the instant at which the Zone ceases following the Rule, and 
the standard time offset from UTC during the interval between the 
start and end time.  For all times within the interval that the 
Zone follows the Rule, the DST offsets and DST transitions defined 
by the Rule apply to the Zone.

The Zone might or might not incur a standard time transition 
and/or a DST transition at the instant when the zone begins or 
ceases to follow the Rule.  To figure out whether or not there are 
standard time transitions or DST transitions at these instants, 
one needs to determine both the standard time offset from UTC, and 
the DST offset from standard time, at the instants just before and 
just after the Zone begins to follow the Rule, or ceases to follow 
the Rule; any differences in the offsets imply that there is a 
transition at that instant.

--apb (Alan Barrett)