According to ECMA-402, section 6.4, Intl.DateTimeFormat uses the host environment’s current time zone if no time zone is provided to the constructor. Bug 796523 enabled the implementation of Date objects to track the operating system's time zone, but DateTimeFormat doesn't do that yet. As a result, toString and toLocaleString (with no time zone specified) can produce output reflecting different time zones at the same time. This can be observed, for example, with the test case at http://norbertlindenberg.com/ecmascript/DateTest.html See also https://mail.mozilla.org/pipermail/es-discuss/2013-July/032087.html
Would fixing this bug also provide the current OS timezone via Intl.DateTimeFormat().resolvedOptions().timeZone ? Also a related side question, is there an event of some sort when the system timezone changes?
Andre, this also seems fixed now, am I correct?
IIRC the DateTest.html page (before it went 404) also tested the behaviour when the time zone is manually changed during the life time of a web page. We still don't have that part correctly implemented (bug 1343826), so we probably should keep this bug open.