The numbers are already arbitrary. Even within the US, where the east and west are 'only' three hours apart there are many who believe that only their timezone

The numbers are already arbitrary. Even within the US, where the east and west are 'only' three hours apart there are many who believe that only their timezone matters. For conveying when in time something happened or will happen, using UTC properly is good. There is one singular number that can make sense to everyone. For all other things people perform a lookup, either in their mind or in some utility (paper or program) already. Times for accounting (arbitrary numbers that presently have local offsets) could benefit from having the offset, and the need for specifying /what/ that offset is, removed. There is no reason we //need// 9am to mean 'morning', we can use morning for that. Any 'time' that isn't a hard number is a loose social reference for local meanings.