diff options
Diffstat (limited to 'thirdparty/icu4c/common/wintz.cpp')
| -rw-r--r-- | thirdparty/icu4c/common/wintz.cpp | 24 |
1 files changed, 20 insertions, 4 deletions
diff --git a/thirdparty/icu4c/common/wintz.cpp b/thirdparty/icu4c/common/wintz.cpp index 580cedadb6..ebf31650c2 100644 --- a/thirdparty/icu4c/common/wintz.cpp +++ b/thirdparty/icu4c/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); } |