I’m listening to Carlo Rovelli’s book, “Reality is not what it seems”, and I’m still hung up on the concept of the “extended now”. The way he describes it, everything being relativistic, there’s no such thing as an “objective” point of view. So, if I understand his take on your example, we’re not seeing the sun as it was 8 minutes ago. It simply takes 8 minutes for the sun’s “now” to reach us.
UTC is a global clock. If I show the time in Sydney and the time here in UK, Sydney will be 150ms behind to me, and 150ms ahead to a viewer in Sydney, but I know the distance therefore I know if the clocks are in sync.
The time dilation between the two places is on the order of femtoseconds/second, a millionth of a clock cycle of a cpu.
CET and EST are also the same time in Miami as in Singapore. That's just how time zones work. The distinction with UTC is that it isn't tied to any particular physical location, it's not the "time zone" for anywhere.
But UTC is ultimately defined by consensus. We need a reference clock and we need to be able to measure or estimate our skew in order to sync to it.
GPS provides the correct time anywhere on the planet, it's a universal (as far as earth goes) clock, accurate to 10 nanoseconds (3m). The different reference planes that opposite sides of the planet (at the equator) gives you a precision of femtoseconds (micrometers at light speed) so makes no difference there.
The quality of GPS receiver of course is important, it may reduce your precision to to 100ns or even 1000ns, but it will give you the right time. Someone equidistant to two GPS synced clocks will see them both at the same time (within a microsecond)
