new time conversion interface

[Paul Hill]

Nathan has provided an interesting list of ambiguities, but I see a few problems
with the list.
Nathan Myers wrote:

> 0.  Unambiguous time, e.g. 04:30 morning of a time change
>
>   t0=123456789
>   offset 0: 0 sec; wall-clock offset: -3600 sec;
>      interpretation: unambiguous; confidence: certain
>   offset 0: -3600 sec; wall-clock offset: -3600 sec.
>      interpretation: suggested substitute; confidence: doubtful
>
> 1. Spring ambiguity, enter 02:30 when it doesn't exist because
>    civil time proceeded 01:59:59 -> 03:00:00.  (Or 02:00:00 to
>    03:00:01?  I don't know.)
>
>   t0=123456789
>   offset 0: 0 sec; wall-clock offset: -3600 sec.
>      interpretation: suggested substitute; confidence: doubtful
>
> 2. Spring ambiguity, enter 02:30 when it doesn't exist because
>    civil time proceeded 01:59:59 -> 03:00:00 (or whatever).
>
>   t0=123456789
>   offset 0: 0 sec; wall-clock offset: 0 sec.
>      interpretation: official choice; confidence: Nominally unambiguous
>   offset 1: -3600 sec; wall-clock offset: 0 sec.
>      interpretation: suggested substitute; confidence: doubtful
>
> 3. Autumn ambiguity, enter 01:30 on morning when civil time proceeds
>    from 01:59:59 to 01:00; is it the first or second 01:30 event?
>
>   t0=123456789
>   offset 0: 0 sec; wall-clock offset: 3600 sec.
>      interpretation: ambiguous choice; confidence: equal alternative
>   offset 1: 3600 sec; wall-clock offset: 0 sec.
>      interpretation: ambiguous choice; confidence: equal alternative
>
> 4. Autumn, enter 02:30, same morning as above; did they mean the
>    official 02:30, or did they mean the second 01:30 because they
>    failed to reset their clock?
>
>   t0=123456789
>   offset 0: 0 sec; wall-clock offset: 0 sec.
>      interpretation: official choice; confidence: Nominally unambiguous
>   offset 1: 3600 sec; wall-clock offset: 0 sec.
>      interpretation: unofficial choice; confidence: Possible alternative

"Because they failed to reset their clock"!  That possibility could apply to all
times both DLS and non-DLS during all days of the year (or at least during some
fuzzy set period in and around each time change), so the additional entry in #4
(caused by reading from an unofficial clock that wasn't changed) should be the
same as the additional entry in #0 (caused by reading from an unofficial clock
that wasn't changed). In the #0 your second possibility is "doubtful ...
substitution", in #4 you have "possible ... unofficial".  I don't see them as
different.  What are you trying to suggest by having so many catagories? How can
you really differentiate between them?

At a minimum this appears to suggest that there is a redundancy in your proposed
catagories, but maybe I don't completely understand the use of the two offsets,
which also differ between the two possibilities, but I can't see why they would.

Speaking of ambiguity, could you explain what your are really trying to capture
in #1 and #2, because your descriptions are of the same circumstance.  Maybe you
meant #1 is one half hour after the time change ("2:30" is a Standard Time) and
#2 is one half-hour before the time change ("2:30" is a DLS Time). If so, these
are also the result of reading a clock that wasn't reset correctly and putting
that value in the tm struct, so why are your return results different by more
than just a sign in the offset.

This seems to leave us with only one other possibility, #3, the one hour of time
that really does correctly exist twice on a wall clock running in both Standard
and DLS time correctly.

thanks,
-Paul Hill