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ICU-21465 Fix Windows Time Zone offset when Automatic DST setting is Off #1539

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Jan 26, 2021
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ICU-21465 Fix Windows Time Zone offset when Automatic DST setting is Off #1539

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24 changes: 20 additions & 4 deletions icu4c/source/common/wintz.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
}
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