Because the Earth is a sphere, cities around the world uses different clocks.
When London is at 9 a.m., New York is at 4 am and Tokyo is at 6pm.
It's called timezone.

With the "World Clock" in iOS preinstalled application "Clock", you can know the current time of any place in the world. But if you want to know the time of another place at 9 p.m. in this place, the calculation is not easy.
When is noon at a place, what time is it in another place?

With this application you can easily compare the times in places you specified around the world with each other.