| diff --git a/source/common/wintz.cpp b/source/common/wintz.cpp |
| index 580cedad..ebf31650 100644 |
| --- a/source/common/wintz.cpp |
| +++ b/source/common/wintz.cpp |
| @@ -124,10 +124,26 @@ uprv_detectWindowsTimeZone() |
| // No way to support when DST is turned off and the offset in minutes is not a multiple of 60. |
| if (utcOffsetMins % 60 == 0) { |
| char gmtOffsetTz[11] = {}; // "Etc/GMT+dd" is 11-char long with a terminal null. |
| - // Note '-' before 'utcOffsetMin'. The timezone ID's sign convention |
| - // is that a timezone ahead of UTC is Etc/GMT-<offset> and a timezone |
| - // behind UTC is Etc/GMT+<offset>. |
| - int ret = snprintf(gmtOffsetTz, UPRV_LENGTHOF(gmtOffsetTz), "Etc/GMT%+ld", -utcOffsetMins / 60); |
| + // Important note on the sign convention for zones: |
| + // |
| + // From https://en.wikipedia.org/wiki/Tz_database#Area |
| + // "In order to conform with the POSIX style, those zone names beginning with "Etc/GMT" have their sign reversed |
| + // from the standard ISO 8601 convention. In the "Etc" area, zones west of GMT have a positive sign and those |
| + // east have a negative sign in their name (e.g "Etc/GMT-14" is 14 hours ahead of GMT)." |
| + // |
| + // Regarding the POSIX style, from https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html |
| + // "The offset specifies the time value you must add to the local time to get a Coordinated Universal Time value." |
| + // |
| + // However, the Bias value in DYNAMIC_TIME_ZONE_INFORMATION *already* follows the POSIX convention. |
| + // |
| + // From https://docs.microsoft.com/en-us/windows/win32/api/timezoneapi/ns-timezoneapi-dynamic_time_zone_information |
| + // "The bias is the difference, in minutes, between Coordinated Universal Time (UTC) and |
| + // local time. All translations between UTC and local time are based on the following formula: |
| + // UTC = local time + bias" |
| + // |
| + // For example, a time zone that is 3 hours ahead of UTC (UTC+03:00) would have a Bias value of -180, and the |
| + // corresponding time zone ID would be "Etc/GMT-3". (So there is no need to negate utcOffsetMins below.) |
| + int ret = snprintf(gmtOffsetTz, UPRV_LENGTHOF(gmtOffsetTz), "Etc/GMT%+ld", utcOffsetMins / 60); |
| if (ret > 0 && ret < UPRV_LENGTHOF(gmtOffsetTz)) { |
| return uprv_strdup(gmtOffsetTz); |
| } |
