> timezones have precise boundaries that cover the whole earth, no?
Not only is that not true, but there are places on the earth where one country may think the timezone is X, but in another country it is illegal to claim that the timezone is X.
This happens most notably with China, which defines the entire country to be in a single timezone, and is engaged in multiple border disputes.
Similarly for many disputed territories in Antarctica, with the added fun of "the south pole region is special". But of course that is a very marginal topic.
A triple (Lat/Lon/UTC Date) <-> TimeZone at the very least.
Time zones are political | * social* especially in the twilight zone between two otherwise well defined zones.
If a country is mostly in TimeZone X then often an extrusion that goes well into TimeZone Y will remain with clocks aligned to TZ-X unless some local political change is made (to be more or less sensible).
and note that cities have changed time zones (not daylight savings changes, actual changes to core time zone) in the past and will almostly certainly do so again in the future.
