[Datetime-SIG] What's are the issues?

[Łukasz Rekucki]

On 29 July 2015 at 19:23, Alexander Belopolsky
<alexander.belopolsky at gmail.com> wrote:
> On Wed, Jul 29, 2015 at 1:05 PM, Lennart Regebro <regebro at gmail.com> wrote:
>>> Not so.  It can easily (albeit rarely) arise in cases of pure timezone
>>> conversions.  Conversions have nothing to do with arithmetic.  If I
>>> convert a time in zone X to a time in zone Y that lands in an
>>> ambiguous span in zone Y, without the conversion _also_ setting an
>>> is_dst flag in the destination, it's impossible to reliably convert
>>> the time in zone Y back to zone X again.
>>
>> No, you just consistently pick one of the two times in both
>> conversions to and from and it will roundtrip just fine.
>
> This may work when both conversions have an ambiguity, but in the real
> life this does not happen.
>
> Assume the time difference between X and Y in 12 hours, and naturally
> Y does not have a DST transition in the middle of the day.
>
> Let's start the conversion:
>
> "01:30 X/DST" -> "13:30 Y" ->  "01:30 X/DST"
>
> So far so good.  We ignore DST in the first transition, get
> unambiguous time in the Y zone and get the original X time in the
> second transition.
>
> However, we we start with "01:30 X/STD", we must get
>
> "01:30 X/STD" -> "13:30 Y"

I think I'm lost at this point. Assume Y = UTC (fulfills all your
assumptions for some -12 zone). Why would 01:30 X/DST and 01:30 X/STD
convert to the same point in time ?

-- 
Łukasz Rekucki