This puts all use of system time_t functions in one place, instead of spreading it out. Its implementation improves on what was formerly done in V4 Date's offset calculations, while simplifying them and eliminating most of the #if-ery. Add four more test-cases to tst_qqmlqt::dateTimeConversion(), based on issues seen on MinGW, getting the time-zone wrong due to the failure of localtime_r(); MinGW can use localtime_s, as QLocalTime now does. Revised tst_qqmllocale::timeZoneUpdated()'s conditions. The QEXPECT_FAIL()s have stopped triggering, at least on Darwin, and the issue isn't that Date.timeZoneUpdated() wasn't working, it's that (now only on Android and Windows) we don't have a way to set the system time-zone referenced by the system functions that QLocalTime calls to get time-zone offsets. Setting the TZ environment variable only works on faithful POSIX implementations. Pick-to: 6.4 Fixes: QTBUG-85149 Fixes: QTBUG-95993 Fixes: QTBUG-102971 Change-Id: I7bc983b9fd7167e3bab3db41dbc1c6f4a78665b9 Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>