accurate matching of our time zone to the system's zone.  This method is
able to distinguish Antarctica/Casey from Australia/Perth, as in Chris
K-L's recent example; and it is not materially slower than before, because
the extra checks generally don't get done against very many time zones.

It seems possible that with this test we'd be able to correctly identify
Windows timezones without looking at the timezone name, but I do not
have the ability to try it.