In the future, the world will use UTC and sunrise and sunset will happen at different times, relative to where you are.
Edit: We basically just need a logical clock though we may face problems reconciling this logical clock in a distributed space/time system.
"It's 10:00 here in London. I need to call someone in New York, what time is it there?" "10:00, same as it is everywhere in the world." "So... is now a good time to call them?" "I have no idea."