[stoney]

If everyone used UTC you'd still have many of the same problems, just in reverse. For example, you can reasonably guess that in any country, an office will be staffed on weekdays from roughly 9-5 local time. If we all operated on UTC, then you'd need to look up in a big table what times the Sydney office operated.

It is useful to have a shared understanding of where in the day you expect an event to happen. E.g. 1pm is going to be around lunchtime whether you are in Sydney or New York.

Also, dates have exactly the same problems as time zones - the date starts at midnight local time. If lcoal time changes, then for part of the day, so does the date.