Re: [Boost-users] [date_time] dst_local_start_time

Subject: Re: [Boost-users] [date_time] dst_local_start_time
From: Roland Bock (rbock_at_[hidden])
Date: 2008-11-05 15:25:16

Hi,

forgot to write some kind of conclusion after my code sample:

My guess: The dst_local_start_time() function returns a ptime object 
which actually represents wall time at the location (Zurich in your case).

I would say that 2007-Mar-25 02:00:00 is the start of the DST just like 
2007-Mar-25 03:00:00, because those two times are exactly the same. It's 
not like "when the clock reaches 02:00, we've reached end of wintertime, 
then we switch to 03:00 and have beginning of summertime". On that day, 
in the infinitesimal time interval when it is 02:00, it is also 03:00.

So, personally, I would say, the confusing part is that local times are 
represented by ptimes in some cases (see also the example I sent). The 
result is OK, though.

If you look at the dst_local_end_time(), it is consistent with the start 
function: In both cases, it returns the time which would be shown on the 
clock without the switch.

Regards,

Roland

Roland Bock wrote:
> Hi,
> 
> I had a similar problem in getting the current wall time in New York. I 
> wrote the following working function and added a bunch of comments, 
> since the library is counter-intuitive here IMHO:
> 
> 
> // Get local time of day, based on time zone
> const boost::posix_time::time_duration 
> DateTime::localizedTimeOfDayAsDuration(const 
> boost::local_time::time_zone_ptr timeZone)
> {
>    /* This looks a bit perverted (and it is in my eyes :-))
> 
>       Calling time_of_day on a local_date_time object implicitly turns 
> it into a UTC ptime object first, thus yielding UTC time_of_day instead 
> of the wall time (I do not really understand why it works this way, but 
> it does).
> 
>       Calling local_time on a local_date_time object returns a ptime 
> object which in this case represents wall time within the given time 
> zone (again, I don't know why the behavior is this way, but it is).
>    */
>    return 
> local_microsec_clock::local_time(timeZone).local_time().time_of_day();
> }
> 
> Please let me know if that helped or if you found a better 
> solution/explanation.
> 
> Regards,
> 
> Roland
> 
> Jean-Pierre Bergamin wrote:
>> Hello
>>
>> I'm confused about the returned time of the dst_local_start_time() 
>> function. First of all I don't understand why a ptime is returned and 
>> not a local_date_time, buy anyway.
>>
>> The ptime I get returned for the timezone Europe/Zurich for 2007 is: 
>> 2007-Mar-25 02:00:00
>> Assuming that this is a UTC time, the local time in Zurich would then 
>> be 2007-Mar-25 04:00:00 (because during summertime, Zurich is 2h ahead 
>> of UTC), but the correct local dst start time would be 2007-Mar-25 
>> 03:00:00 (because the clocks are shifted from 2 to 3).
>> To correctly calculate the dst start time, I have to add the 
>> dst_offset to the returned time_of_day.
>>
>> The returned ptime 2007-Mar-25 02:00:00 seems to be the endtime of 
>> wintertime as a local time.
>>
>> time_zone_ptr zh_tz = tz_db.time_zone_from_region("Europe/Zurich");
>> ptime zh_dst_start_time = zh_tz->dst_local_start_time(2007);
>> std::cout << zh_dst_start_time << std::endl; // 2007-Mar-25 02:00:00
>> local_date_time zh_dst_local_start_time = 
>> local_date_time(zh_dst_start_time, zh_tz);
>> std::cout << zh_dst_local_start_time << std::endl; // 2007-Mar-25 
>> 04:00:00 CEST which is wrong!
>>
>> ptime zh_dst_start_time_ok = zh_dst_start_time + zh_tz->dst_offset();
>> local_date_time zh_dst_local_start_time_ok = 
>> local_date_time(zh_dst_start_time_ok.date(), 
>> zh_dst_start_time_ok.time_of_day(), zh_tz, true);
>> std::cout << zh_dst_local_start_time_ok << std::endl; // 2007-Mar-25 
>> 03:00:00 CEST which is correct
>>
>> So what time exactly is local_start_time() returning?
>> Is this a bug or am I missing something?
>>
>>
>> Regards
>>
>> James
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