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

From: Jeff Garland (jeff_at_[hidden])
Date: 2006-10-27 23:10:24

Jacob Anawalt wrote:
> Jeff Garland wrote:
>> Well, this is really just convention at some level.  
> 
> Right. POSIX.1 (IEEE Std 1003.1) convention as far as I am able to find.

Sorry for the delayed response...travel and such.  Well, I spoke a bit to 
losely, it isn't just convention it's in the spec.

> Unless I've found the wrong POSIX.1 documentation I consider the fact 
> that posix_time_zone expects negative values for offsets west of the
> Prime Meridian or the fact that it claims to use IEEE Std 1003.1 
> timezone strings to be a bug.

Anyway, if you go to http://www.unix.org/version3/ieee_std.html and register 
you can read the official spec.  Here's the excerpt of the key parts:

TZ
     This variable shall represent timezone information....

The expanded format (for all TZ s whose value does not have a colon as the 
first character) is as follows:

     stdoffset[dst[offset][,start[/time],end[/time]]]
...

offset
     Indicates the value added to the local time to arrive at Coordinated
     Universal Time. The offset has the form:

         hh[:mm[:ss]]

The minutes ( mm) and seconds ( ss) are optional. The hour ( hh) shall be 
required and may be a single digit. The offset following std shall be required

....

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 
'+' ).

> It would be nice to be able to use the TZ environment variable as the 
> argument to the posix_time_zone constructor if TZ existed and did not l
> have a leading colon.

So, after staring at this spec for awhile, it looks to me that this is a major 
bug in posix timezone :-( For whatever reason, posix has the utc offset 
definitions inverted from the way it actually is...so you always have to 
invert the sign of the offset to get the actual utc offset.

To be 100% sure, I'll need a bit more info. What OS are you using, what 
timezone are you in, and what is TZ set to?  Assuming it's unix, what do 
'date' and 'date -u' give you?

And then what I assume you are doing exactly in date-time.  Based on the first 
email you sent I imagine it's something like:

shared_ptr<time_zone> tz(new posix_time_zone("MST7MDT,M4.1.0,M10.5.0"));
local_date_time ldt1 = local_sec_clock::local_time(tz);

Jeff