Hi, I think that posix_time_zone in Boost.DateTime is using the wrong sign for offsets from GMT: According to the documentation, posix_time_zone is modeled after IEEE Std 1003.1. The documentation gives several examples, for instance <cite> "MST-7" This zone is as simple as it gets. This zone lies seven hours west of GMT and has no daylight savings. </cite> But all other sources for IEEE Std 1003.1 I found so far say that negative offsets indicate *east* of GMT, see for instance: http://www.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html <cite> If 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 '+' ). </cite> http://tools.ietf.org/html/draft-ietf-dhc-timezone-01 <cite> The format of the IEEE 1003.1 POSIX timezone specification is defined as follows:[...]If preceded by a '-', the timezone is east of the Prime Meridian, otherwise it is west ('+' is optional) </cite> http://www.twinsun.com/tz/tz-link.htm <cite> Numeric time zone abbreviations typically count hours east of UTC, e.g., +09 for Japan and -10 for Hawaii. However, the POSIX TZ environment variable uses the opposite convention. For example, one might use TZ="JST-9" and TZ="HST10" for Japan and Hawaii, respectively. </cite> IMO, the implementation or the documentation should be changed. Personally, I'd prefer a change in the implementation, even though this would be a rather nasty breaking change. Regards, Roland