Noon cannot be everywhere at once

Why Is It a Different Day Somewhere Else?

Clocks are set by the Sun, and the Sun is only overhead in one place at a time.

Twenty-four stripes, one for each hour of the turn.

Step 1 of 5

A day is one turn of the Earth.

The Earth spins once in twenty-four hours, so every place passes under the Sun once. Noon is not a moment — it is a place.
Step 2 of 5

Dividing the turn gives the zones.

Three hundred and sixty degrees divided by twenty-four hours is fifteen degrees an hour. Each stripe of fifteen degrees keeps one clock time.
Step 3 of 5

Before railways, every town kept its own noon.

Clocks were set by the local Sun, so a town twenty miles east was minutes ahead. Timetables made that unworkable overnight.
Step 4 of 5

The zones bend around countries.

The arithmetic gives stripes; the politics gives the real map

The stripes are not straight. Borders pull them about so a country can share one time, and a few zones sit half an hour off the rest.
Step 5 of 5

Somewhere the date has to change.

Go far enough east adding hours and you come back around. The line down the Pacific is where the arithmetic is forced to jump a whole day.

Two islands in the Pacific sit a few miles apart and twenty-five hours away from each other.

The short version

Time zones divide the Earth's twenty-four-hour turn into stripes, so clocks stay roughly in step with the Sun.

Work out the time in a city eight zones away by adding eight hours. If it goes past midnight, it is tomorrow there.