Hi Leon,

 

Yes, but the problem is that the offset for Berlin in Boost date_time_zonespec.csv is +01, not -01

Yeah, I got that, just wanted to confirm that you read the Posix docs correctly.

I tried your code with older Boost (1.62) on my Mac and got the same string you put in comment (CET+01CEST+01,M3.5.0/02:00,M10.5.0/03:00) and if I feed this string to my TZ, for sure the output of date command is two hours off.

It seems that to_posix_string() is broken. I think that the author of the library may have incorrectly interpreted the standard; unfortunately I don't have a copy of ISO/IEC 9945-1 at hand so I can't confirm that. But a quick Internet search for examples shows strings such as PST8PDT7,M4.1.0/2:00:00,M10.5.0/2:00:00 (US/Pacific), which indicate that the timezones west of GMT have positive offsets, which also means that Central Europe should have negative offsets. Also, the Boost documentation (https://www.boost.org/doc/libs/1_69_0/doc/html/date_time/local_time.html#date_time.local_time.posix_time_zone) states (Introduction, near the top):

    'std' specifies the abbrev of the time zone. 'offset' is the offset from UTC

Which would explain the incorrect signs ... in Posix it's the other way around, the offset is really the offset of UTC from the local time.

Of course, all that will not help you, the results will still be incorrect. The only course of action I can recommend is to check what exactly ISO/IEC 9945-1 states, and if above is correct, fill a bug report at Boost. Unfortunately I can't think of any short term solution, other than looking for a different library.

Cheers,

Leon

 

n  Mats

 

Från: Boost-users [mailto:boost-users-bounces@lists.boost.org] För Leon Mlakar via Boost-users
Skickat: den 28 december 2018 22:18
Till: boost-users@lists.boost.org
Kopia: Leon Mlakar
Ämne: Re: [Boost-users] Question on Posix timezone string

 

On 28.12.2018 21:34, Mats Webjörn/WIKAB via Boost-users wrote:

I’m using Boost 1.66 and use the following code to get a Posix string to set my uCLinux TZ environment variable to get a local time from an UTC time

 

#include <boost/date_time/local_time/tz_database.hpp>

#include <boost/date_time/local_time/local_time.hpp>

 

tz_database tz_db;

tz_db.load_from_file("./date_time_zonespec.csv");

time_zone_ptr nyc = tz_db.time_zone_from_region("Europe/Stockholm");

auto s = nyc->to_posix_string();  // "CET+01CEST+01,M3.5.0/02:00,M10.5.0/03:00"

setenv("TZ", s.c_str(), 1);

 

The odd thing is that my localtime becomes incorrect, and is actually 1 hr wrong on the opposite side of UTC.

 

But, when I read https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html it says

 

“The offset specifies the time value you must add to the local time to get a Coordinated Universal Time value”.

 

Not add to UTC time, which seems like the Boost Posix string assumes !!!!!

Which explains why 16:00 UTC give 15:00 CET instead of 17:00 which is the correct value, when using localtime_r().

I think this document is correct, and that POSIX systems actually add the TZ offset to the displayed local time to yield UTC, unlike to ISO 8601 where offset is subtracted from the local time (or, change the sign of the offset, then add :-) in the time string, to get the UTC .

Five minutes past noon on one winter day in Berlin would thus be 12:05:00 with TZ offset -01 on Posix system and 12:05:00+01 in ISO 8601 textual presentation.

Cheers,

Leon