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Bug 1533328 - Part 8: Add comments explaining when <ctime> and ICU can return a different default time zone. r=jorendorff

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Differential Revision: https://phabricator.services.mozilla.com/D41376

--HG--
extra : moz-landing-system : lando
This commit is contained in:
André Bargull 2019-08-20 09:09:19 +00:00
parent df1430cb05
commit 9694d385a0
1 changed files with 23 additions and 1 deletions
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24
js/src/vm/DateTime.h
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@ -238,7 +238,8 @@ class DateTimeInfo {
/**
* The offset in seconds from the current UTC time to the current local
* standard time (i.e. not including any offset due to DST).
* standard time (i.e. not including any offset due to DST) as computed by the
* operating system.
*
* Cached because retrieving this dynamically is Slow, and a certain venerable
* benchmark which shall not be named depends on it being fast.
@ -247,6 +248,27 @@ class DateTimeInfo {
* system time zone to attempt to keep this reasonably up-to-date. If
* temporary inaccuracy can't be tolerated, JSAPI clients may call
* JS::ResetTimeZone to forcibly sync this with the system time zone.
*
* In most cases this value is consistent with the raw time zone offset as
* returned by the ICU default time zone (`icu::TimeZone::getRawOffset()`),
* but it is possible to create cases where the operating system default time
* zone differs from the ICU default time zone. For example ICU doesn't
* support the full range of TZ environment variable settings, which can
* result in <ctime> returning a different time zone than what's returned by
* ICU. One example is "TZ=WGT3WGST,M3.5.0/-2,M10.5.0/-1", where <ctime>
* returns -3 hours as the local offset, but ICU flat out rejects the TZ value
* and instead infers the default time zone via "/etc/localtime" (on Unix).
* This offset can also differ from ICU when the operating system and ICU use
* different tzdata versions and the time zone rules of the current system
* time zone have changed. Or, on Windows, when the Windows default time zone
* can't be mapped to a IANA time zone, see for example
* <https://unicode-org.atlassian.net/browse/ICU-13845>.
*
* When ICU is exclusively used for time zone computations, that means when
* |ENABLE_INTL_API && !MOZ_SYSTEM_ICU| is true, this field is only used to
* detect system default time zone changes. It must not be used to convert
* between local and UTC time, because, as outlined above, this could lead to
* different results when compared to ICU.
*/
int32_t utcToLocalStandardOffsetSeconds_;