This mapping is used to look up abbreviation
to the possible regional zones.
It looks like this:
([ "CET": ({ "Europe/Stockholm", [...] }),
"CST": ({ "America/Chicago", "Australia/Adelaide", [...] }),
[...] }),
Note this: Just because it's noted "CST" doesn't mean it's a
unique timezone. There is about 7 *different* timezones that
uses "CST" as abbreviation; not at the same time,
though, so the DWIM routines checks this before
it's satisfied. Same with some other timezones.
For most timezones, there is a number of region timezones that for the
given time are equal. This is because region timezones include rules
about local summer time shifts and possible historic shifts.
The YMD.parse functions can handle timezone abbreviations
by guessing.