[Issue 1436] New: std.date.getLocalTZA() returns wrong values when in DST under Windows

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=1436

           Summary: std.date.getLocalTZA() returns wrong values when in DST
                    under Windows
           Product: D
           Version: 1.020
          Platform: All
        OS/Version: Windows
            Status: NEW
          Keywords: patch, wrong-code
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: bugzilla at digitalmars.com
        ReportedBy: deewiant at gmail.com


Currently, getLocalTZA() does the following:
--
r = GetTimeZoneInformation(&tzi);
switch (r) {
        case TIME_ZONE_ID_STANDARD:
        case TIME_ZONE_ID_DAYLIGHT:
        case TIME_ZONE_ID_UNKNOWN:
        t = -(tzi.Bias + tzi.StandardBias) * cast(d_time)(60 * TicksPerSecond);
        break;

        default:
        t = 0;
        break;
}

return t;
--
As can be seen, it always uses the StandardBias field, as long as
GetTimeZoneInformation doesn't result in an error.

However, this is incorrect. When TIME_ZONE_ID_DAYLIGHT is returned, the
DaylightBias field should be used, as this indicates that the system is
currently using Daylight Savings Time. StandardBias is meant to be used only
when the system is not in DST, as documented at
http://msdn.microsoft.com/library/en-us/sysinfo/base/gettimezoneinformation.asp
and http://msdn2.microsoft.com/en-us/library/ms725481.aspx

One possible working version follows:
--
r = GetTimeZoneInformation(&tzi);
switch (r) {
        case TIME_ZONE_ID_STANDARD: t = tzi.Bias + tzi.StandardBias; break;
        case TIME_ZONE_ID_DAYLIGHT: t = tzi.Bias + tzi.DaylightBias; break;
        case TIME_ZONE_ID_UNKNOWN:  t = tzi.Bias; break;

        default:
        t = 0;
        break;
}

return -t * cast(d_time)(60 * TicksPerSecond);


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