[Mono-dev] Time zone problems with DateTime.Parse (patch and bug)

[Atsushi Eno]

It depends on how 1) underlying localtime() works on *nix environment
and 2) GetTimeZoneInformation() works on Windows environment.

Atsushi Eno

Harry Holt wrote:
> Hmmm... Will the changes in daylight saving time for 2007+ (
> http://aa.usno.navy.mil/faq/docs/daylight_time.html) have any affect on 
> this?
> 
> ... HH
> 
> On 9/7/05, Atsushi Eno <atsushi at ximian.com> wrote:
>> Hi,
>>
>>>>>> Test case:
>>>>>> http://bugzilla.ximian.com/show_bug.cgi?id=75985
>>>>>>
>>>>>> I think the problem here is that the internal DateTime(bool,long)
>>>>>> constructor calls tz.GetUtcOffset(this) with the UTC time to get the
>>>>>> timezone offset before applying it to get local time, but that 
>> function
>>>>>> expects a local time to determine if DST is active. A bit of a
>>>>>> chicken-and-egg problem, perhaps... ;)
>>>>>
>>>>> Agreed. But it seems possible to avoid that problem. Can you please
>>>>> try attached patch? I don't have sane Unix environment and I'm
>>>>> not in such region that has summer time ;-) It would be even nicer
>>>>> if you try the entire corlib Nunit tests as well.
>>>> Cool, that's definitely closer! Unfortunately it's still a bit off;
>>>> during the doubled hour in the DST transition it's an hour off in the
>>>> wrong direction. Using output from my test proggy on that bug:
>>> Okk, based on your help, I found some things. The most important one
>>> is that there are different basis of TimeZone: UTC and local time
>>> (there might be other patterns). now I think we need our own
>>> TimeZone data store, that would store timezone names as well.
>>>
>>> At least DateTime.ToLocalTime() will be rewritten just to invoke
>>> TimeZone.CurrentTimeZone.ToLocalTime(this).
>>>
>>> So, I'll revisit here later. Thanks for all your help, Brion :-)
>> After some attempt (I think) I could fix TimeZone.ToLocalTime() for
>> PST/PDT (the attached patch is for bug #75985). However, I have no
>> better idea than that it just fixes offsets in that timezone, as
>> I mentioned my concern about the basis difference (UTC or localtime).
>>
>> So I will have to dig into POSIX timezone design in depth.
>> And apart from the matter above, I will still have to extend our
>> locale-builder to support TimeZone name.
>>
>> Atsushi Eno
>>
>>
>> Index: System/TimeZone.cs
>> ===================================================================
>> --- System/TimeZone.cs (revision 49638)
>> +++ System/TimeZone.cs (working copy)
>> @@ -108,27 +108,33 @@
>>
>> public virtual DateTime ToLocalTime (DateTime time)
>> {
>> -// return time + GetUtcOffset (time);
>> - TimeSpan offset = GetUtcOffset (time);
>> -
>> - if (offset.Ticks > 0) {
>> - if (DateTime.MaxValue - offset < time)
>> + DaylightTime dlt = GetDaylightChanges (time.Year);
>> + TimeSpan utcOffset = GetUtcOffset (time);
>> + if (utcOffset.Ticks > 0) {
>> + if (DateTime.MaxValue - utcOffset < time)
>> return DateTime.MaxValue;
>> - } else if (offset.Ticks < 0) {
>> - // MS.NET <http://MS.NET> fails to check validity here
>> - // - it may throw ArgumentOutOfRangeException
>> - /*
>> - if (DateTime.MinValue - offset > this)
>> - return DateTime.MinValue;
>> - */
>> + //} else if (utcOffset.Ticks < 0) {
>> + // LAMESPEC: MS.NET <http://MS.NET> fails to check validity here
>> + // it may throw ArgumentOutOfRangeException.
>> }
>> -
>> - DateTime lt = new DateTime (time.Ticks + offset.Ticks);
>> - TimeSpan ltoffset = GetUtcOffset (lt);
>> - if (ltoffset != offset)
>> - lt = lt.Add (ltoffset.Subtract (offset));
>>
>> - return lt;
>> + DateTime local = time.Add (utcOffset);
>> + if (dlt.Delta.Ticks == 0)
>> + return local;
>> +
>> + // FIXME: check all of the combination of
>> + // - basis: local-based or UTC-based
>> + // - hemisphere: Northern or Southern
>> + // - offset: positive or negative
>> +
>> + // PST should work fine here.
>> + if (local < dlt.End && dlt.End.Subtract (dlt.Delta) <= local)
>> + return local;
>> + if (local >= dlt.Start && dlt.Start.Add (dlt.Delta) > local)
>> + return local.Subtract (dlt.Delta);
>> +
>> + TimeSpan localOffset = GetUtcOffset (local);
>> + return time.Add (localOffset);
>> }
>>
>> public virtual DateTime ToUniversalTime (DateTime time)
>>
>>
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>>
>>
>>
> 
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