to_posix_string() returns invalid timezone string

The following code
`
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();
setenv("TZ", s.c_str(), 1);
`
will set TZ to "CET+01CEST+01,M3.5.0/02:00,M10.5.0/03:00" which will cause Stockholm local time to be 10:00:00 when UTC(GMT/London) time is 11:00:00 which clearly is incorrect.

ANSI/IEEE Std 1003.1 section **8.3 Other Environment Variables** reads for **TZ** variable

Offset Indicates the value added to the local time to arrive at Coordinated Universal Time

[gnu.org](https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html) reads the same
It also says regarding offset
> This is positive if the local time zone is west of the Prime Meridian and negative if it is east.
>

But Boost 1.69 [date-time](https://www.boost.org/doc/libs/1_69_0/doc/html/date_time/local_time.html) says

> A posix_time_zone is unique in that the object is created from a Posix time zone string (IEEE Std 1003.1). A POSIX time zone string takes the form of:
"std offset dst [offset],start[/time],end[/time]" (w/no spaces).
'std' specifies the abbrev of the time zone. 'offset' is the offset from UTC.
"PST-8PDT01:00:00,M4.1.0/02:00:00,M10.1.0/02:00:00"
"PST-8PDT,M4.1.0,M10.1.0"
These two are actually the same specification (defaults were used in the second string). This zone lies eight hours west of GMT …”
>
If I add -8 to a PST local time I will NOT get GMT=UTC time!! It seems like somewhere here (most likely in to_posix_string()) is a huge bug since the Posix/GNU and Boost interpretation of the Posix timezone strings contradict each other.