On Mon, Apr 13, 2015 at 1:24 PM, Chris Barker <chris.bar...@noaa.gov> wrote:
>
>
>> Because of these discontinuities, an equation wall(loc, t) = lt may
>> have 0, 1
>> or 2 solutions.
>>
>
> This is where I'm confused -- I can see how going from "wall" time
> ("local" time, etc) to UTC has 0, 1, or 2 solutions:
>
> One solution most of the time
>
> Zero solutions when we "spring forward" -- i.e. there is no 2:30 am
> on March 8, 2015 in the US timezones that use DST
>
> Two solutions when we "fall back", i.e. there are two 2:30 am Nov 1, 2015
> in the US timezones that use DST
>
> But I can't see where there are multiple solutions the other way around --
> doesn't a given UTC time map to one and only one "wall time" in a given
> timezone?
>
> Am I wrong, or is this a semantic question as to what "wall" time means?
>
You are right about what wall() means, but I should have been more explicit
about knowns and unknowns in the wall(loc, t) = lt equation.
In that equation I considered loc (the geographical place) and lt (the time
on the clock tower) to be known and t (the universal (UTC) time) to be
unknown. A solution to the equation is the value of the unknown (t) given
the values of the knowns (loc and lt).
The rest of your exposition is correct including "a given UTC time maps to
one and only one 'wall time' in a given timezone." However, different UTC
times may map to the same wall time and some expressible wall times are not
results of a map of any UTC time.
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