[Boost-users] [Date_Time] Posix Time Zone offset question

From: Jacob L. Anawalt (jacob+boost_at_[hidden])
Date: 2006-10-25 17:00:20

Hi,

I've started experimenting with Boost.Date_Time's Local Time stuff. 
While using a posix_time_zone object the results were not what I 
expected. I used the POSIX.1 (IEEE Std 1003.1) timezone string suitable 
for use in the TZ environment variable describing my timezone for the 
year 2006:

MST7MDT,M4.1.0,M10.5.0

Date_Time's posix_time_zone object told me that it was already the 26th 
but my wall clock disagrees. :)

The online documentation says "A posix_time_zone is unique in that the 
object is created from a Posix time zone string (IEEE Std 1003.1)." and 
"'offset' is the offset from UTC."

In the sources I've come across the offset part of a POSIX.1 timezone 
string is defined as "the value added to the local time to arrive at 
Coordinated Universal Time" with a not that "[i]f preceded by a '-', the 
timezone shall be east of the Prime Meridian; otherwise, it shall be 
west (which may be indicated by an optional preceding '+' )."

* http://www.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap08.html
* man pages of tzset

Why do I need a negative (-) offset to make posix_time_zone work when 
the time zone is west of the Prime Meridian?

I couldn't find anyone else raising this issue on the list or bug 
tracker, so maybe I'm just up in the night.

Thank-you for the library. I really like some of the concepts in 
Date_Time Posix Time and Gregorian and have high hopes for Local Time stuff.
-- 
Jacob