If we think logically it is rather obvious. The earth spins on an axis and does one revolution every 24 hours. That is one cycle equaling one day.
As the earth turns different parts of the world receive light. think of holding a ball and a torch and shine the torch on the ball. certain p-arts are in the light and others are in the dark.
sunrise as we know it is when the light appears on our horizon and sunset is when it disappears. If we all had the same time zone sunrise would happen at different times for everyone. Midday for another place could be in the middle of the night.
the earth rotates 15 degrees every 60 minutes so one full revolution would be 360 degrees. that would be 24 x 15 = 360 degrees hence why the hour was formed which is 60 minutes.
The earth was divided in 24 time zones of 15 degrees each. Greenwich mean time was named as the starting point so every 15 degree either way would decrease or increase by an hour depending if you went east or west.
image source fotolibra.com and daviddarling.com