Boost C++ Libraries: Ticket #3336: utc offset calculated wrong

component changed; owner set

Steven Watanabe — Tue, 11 Aug 2009 18:59:02 GMT

owner set to az_sw_dude
component None → date_time

Tue, 27 Apr 2010 15:42:04 GMT

The UTC offset in POSIX 1003.1 section 8.3 TZ strings is inverted from that in Olson TZ database strings, which is a cause for confusion. The former treats UTC as relative to the local time, i.e what do we add to the local wall clock to produce the wall clock time in UTC. The latter treats the local time as relative to UTC, as in, what do we add to the time in UTC to get to the local time.

The man pages for tzset(3) seem pretty consistent although some are clearer than others. I like this wording best, myself:

offset: Indicates the value one must add to the local time to arrive at Coordinated Universal Time.

Sun is their own celestial body but they see it the same way:

http://docs.sun.com/source/816-5523-10/appf.htm

USA (Eastern)	EST5EDT
Russia (Moscow)	MST-3MDT

Here also, positive is west of Greenwich, negative is east of it.

As a hacky workaround, the string can be manipulated prior to feeding it to posix_time_zone. Find the first [0-9+-]. If we're not at the end of the string, if '+' change to '-' and vice versa. Otherwise insert '-' before the current position. Something like the following (untested!):

void fliptzoffset(std::string& tzstr) {
  std::string::iterator i = tzstr.begin(), e = tzstr.end();
  while (i != e && !isdigit(*i) && '+'!=*i && '-'!=*i) ++i;
  if (i == e) return;
  if ('-' == *i) *i = '+';
  else if ('+' == *i) *i = '-';
  else tzstr.insert(i, '-');
}

version changed

roman.fietze@… — Tue, 27 Nov 2012 14:11:51 GMT

version Boost 1.38.0 → Boost 1.44.0

This bug doesn't seem to get a lot of attention.

It is still valid for newer versions of the boost library.

Sun, 19 Jan 2014 11:18:14 GMT

Just got bitten by the same bug. Correct behavior:

$ export TZ="PST-8PDT,M3.2.0,M11.1.0"; date -d@1383465600
Sun Nov  3 16:00:00 PST 2013

posix_time_zone maps the same timestamp with the same timezone string to "Sun Nov 3 01:00:00 PST 2013" (16 hours - 1 DST adjustment). Here's another (currently working) link to the POSIX RFC that boost::local_time purports to implement:

http://tools.ietf.org/html/draft-ietf-dhc-timezone-01

The relevant text is at the end of page 2, beginning of page 3.

The boost::local_time documentation and code are both written assuming "-" means "west of Greenwich" and "+" means "east of Greenwich", but the standard clearly states otherwise.

Flipping the sign is pretty easy (I can send a patch, even), but I'm not sure what's Boost's preferred approach for fixing such errors. The flip would break code that depends on the currently broken behavior.

If it's not appropriate to fix the bug in the current API, posix_time_zone2 might be introduced that has the correct sign. Does anybody see other approaches?

version, milestone changed

Sun, 19 Jan 2014 11:31:50 GMT

version Boost 1.44.0 → Boost 1.54.0
milestone Boost 1.40.0 → Boost 1.57.0

I reproduced it in 1.51 and 1.54.