Date/Time: dst_calculator::local_is_dst doesn't deal with DST changeover at end of day

[Matt Adam <matt.adam@…>]

Hello boosters,

dst_calculator::local_is_dst assumes that the hour lost in the "forward" DST changeover (in Spring, when DST begins) fully occurs within a single day. If your time zone rule specifies that DST begins at 23:59:99.999 on October 15th, with a duration of an hour, and you attempt to create a ptime for 00:00:00.000 on October 16th, it will succeed even though it ought to return invalid_time_label.

I admit that this seems like an odd case, and personally I would expect all DST offsets to be in the 0h-3h range. However if you are creating custom time zones based on the win32 time zone information, the GetTimeZoneInformation API returns a TIME_ZONE_INFORMATION struct with values returned a millisecond before midnight. Specifically this happens with Brasilian standard/daylight time.

For now I am "cleaning up" these odd values returned from windows, but boost probably should do this calculation correctly to begin with.

I can provide a repro case if need be, although I think the behaviour of the code in question is pretty clear.

Thanks,

Matt Adam

[jpettiss@…]

DST ended at midnight in Brazil this year on February 22nd. For this timezone, dst_local_end_time() returns the correct value, but a local_date_time constructed from time_t in that timezone returns true for is_dst() for up to an hour after that point. This is due to the logic in dst_calculator::local_is_dst() assuming that if the date is different, then no time distance calculations have to be performed. In this timezone, 2015/02/21 23:30:00 labels two different points, one which happens before the DST change, one which happens after.

  time_zone_ptr tz(new posix_time_zone("BRT3BRST,M10.3.0/0,M2.5.0/0")); //America/Sao_Paulo
  ptime pt = from_time_t( -1 + (time_t)tz->dst_local_end_time() );
  assert( local_date_time(pt, tz).is_dst() ); //dst has not ended yet
  pt = from_time_t( 1 + (time_t)tz->dst_local_end_time() );
  assert( !local_date_time(pt, tz).is_dst() ); //fails
  pt = from_time_t( 3600 + (time_t)tz->dst_local_end_time() );
  assert( !local_date_time(pt, tz).is_dst() ); //dst has ended