Re: Until we become a spacefaring race...
and a significant fraction of the population don't live on the surface of the earth, then most of the race will synchronise their day with the rising and setting of the sun
That makes no sense - the rising and setting of the sun? where - in space?
Once (if) we are a space-faring race we will need to have a time system that is robust enough to handle communication lag over light seconds and minutes, that people can agree on and will mean the same thing where ever everyone happens to be.
What's needed is a far away slow pulsar that can be used as a metronome, or to discover some thing similar to a half-life of reality.
Answers on postcards would almost certainly not be good enough :-)