[tz] Precise meaning of FROM and TO in a rule

Jon Skeet skeet at pobox.com
Mon Jul 7 22:44:17 UTC 2014

Okay, so if I've understood you correctly, the date are always as per the
wall clock. The time *may* be specified in standard or UTC, but that
doesn't affect how the date part is specified. So just to clarify with a
strange example, consider this part of a rule:

Apr Mon>=1 2:00u

So say we're looking for the next transition after 2013-01-01T00:00:00Z,
and the current UTC offset is -8. My interpretation of what you've said is
that we first identify the local date (2013-04-01, which is a Monday) and
then we find the point in time in that 24 hour period at which the UTC time
is 02:00. So:

Monday April 1st, 6pm local (before)
Tuesday April 2nd, 2am UTC

I'm pretty sure that Noda Time wouldn't handle that correctly :(

Could you double check my example before I start fixing code? I think at
the moment I always assume that the time of day and the date are in the
same frame of reference - so in the above example, I suspect I'd find a
transition at Monday April 1st, 2am UTC - a whole day early.

I realize that all of this may be academic at the moment - I doubt that
there are any rules specified which *do* have this sort of effect - but I'd
rather know about bugs sooner rather than later. It might be an interesting
example to try against other implementations, too...


On 7 July 2014 22:57, Paul Eggert <eggert at cs.ucla.edu> wrote:

> Transitions are given in terms of the local time before the transition, as
> otherwise a transition could be self-referential.  A single rule can
> specify multiple transitions, and it's reinterpreted each time it's used.
> Perhaps the following change to zic.8?
> diff --git a/zic.8 b/zic.8
> index fb8f882..bd9dfeb 100644
> --- a/zic.8
> +++ b/zic.8
> @@ -283,6 +283,7 @@ or
>  if the given time is universal time;
>  in the absence of an indicator,
>  wall clock time is assumed.
> +Local time and date is interpreted as of just before the rule takes
> effect.
>  .TP
>  .B SAVE
>  Gives the amount of time to be added to local standard time when the rule
> is in
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