Actually I'm not sure anymore whether NOT specifying a timezone is valid at all.

Even though the Spidermonkey documentation itself states, that it doesn't behave according to the ISO standard, several documents say that specifying the timezone is crucial (either "Z" or "+/-XX:XX").

I also found documents describing the standard which explicitly state that by default the local time is assumed, as JavaScript does.

Either way - there's so much different information and therewith confusion out there, that I highly recommend always specifying the timezone and would consider behaving otherwise as a bug.

Implementations usually don't throw an error but just assume something when the TZ designator is missing, which results in just different meanings. Needless to say that doesn't make the situation any better.