Definition
A time zone is a region that shares one standard clock time, and a time difference is the gap between two zones. They exist because Earth rotates, so each place gets sunlight at a different time. To make daily life and trade easier, the world agreed on standard clocks by slicing the globe into vertical strips measured from the prime meridian.
Earth, the Giant Merry-Go-Round
Picture a dark room. You switch on a flashlight and slowly turn a soccer ball in front of it. The side facing the light is bright as noon. The opposite side is pitch black. Earth does the same thing. It spins once a day (that spin is called rotation), so day and night can't arrive everywhere at the same moment.
Long ago, every town set noon at the moment the sun was highest in its own sky. Then trains arrived, and people could zip between faraway cities. Chaos followed. Every station's clock was off by a few minutes, so building a train schedule or avoiding crashes became dangerous.
So people made one shared rule for the whole planet. They split Earth's 360 degrees across 24 hours of the day and cut the world into vertical strips, so that every 15 degrees of longitude equals a 1-hour time difference.
To Be More Precise: Why the Lines Aren't Straight
If you split time zones perfectly along the longitude lines, it would be the cleanest answer in science. But look at a real world map, and the zone borders zigzag and wiggle. If nature were the only rule, the east and west sides of the same town, or the same company, could end up on different clocks. That would be absurd.
To be more precise, time zones aren't just a natural phenomenon. They follow social agreements and administrative borders that people set for daily life and the economy. Inside one country or city, it's best to share one clock so people can book train tickets and visit government offices without confusion.
Russia and the United States stretch far from east to west, so they use several time zones. China is just as wide, but it uses only one zone, set to Beijing time. So time zones depend not only on longitude but also on national policy and convenience.
What Happens When You Fly Across Time
Fly from Seoul to New York and you have to turn your watch back a whopping 13 to 14 hours. Your body's cells still run on Seoul time and keep sending sleepy signals that say, "It's bedtime." Meanwhile, the world outside is bright as midday. Weird, right?
When your inner body clock doesn't match the local time zone, you get tired and foggy. That feeling is called jet lag. Your organs and hormones need a few days to reset to the new sunlight and meal schedule.
There is also the International Date Line, a line in the middle of the Pacific Ocean where the date changes. It closes the loop on the world's timekeeping. Cross it by plane and, as if by magic, you can gain a day or lose one.
Common misconceptions
Every country's time difference is a whole number of hours, set exactly by longitude.
Some countries and regions use 30-minute offsets, like India, Iran, and central Australia. Nepal even uses a 45-minute offset.
Where you meet it
When it's 12 noon lunchtime in Korea, it's 3 a.m. in London (4 a.m. during British daylight saving time), so most people there are fast asleep.
If you fly from Korea to Los Angeles, you fly for more than ten hours, yet you usually land on the same calendar day you left.
In one sentence
Time zones and time differences are the agreement that keeps the day-and-night gaps between places, caused by Earth's rotation, in good order.