Coasts & borders · 4 min
Time zones are not borders
Why time zones zig-zag, why China has one and why some are half an hour out.
Coasts & borders
The tidy idea
The Earth turns 360° in 24 hours, so every 15° of longitude is one hour. In theory the world splits into 24 neat strips, each 15° wide, counted from Greenwich.
In practice governments choose their own time, and they usually follow their country's or state's borders rather than the strips.
Lesson notes
Everything from the cards above, on one page to come back to.
The tidy idea
The Earth turns 360° in 24 hours, so every 15° of longitude is one hour. In theory the world splits into 24 neat strips, each 15° wide, counted from Greenwich.
In practice governments choose their own time, and they usually follow their country's or state's borders rather than the strips.
Politics bends the lines
China stretches across more than 60° of longitude, enough for four or five tidy zones, but the whole country uses one time, UTC+8, set from Beijing.
Spain sits mostly west of the Greenwich line, yet it keeps the same clock as Germany and Poland, one hour ahead of the UK.
Not always whole hours
Most zones are whole hours from UTC, the world's reference time, but not all. India is UTC+5:30 and Nepal is UTC+5:45.
Daylight saving moves the clocks again for part of the year, on different dates in different countries, and many countries do not use it at all.
Questions
Why does China have only one time zone?
The government set the whole country to Beijing time, UTC+8, in 1949. It keeps one clock across a span that would naturally hold four or five zones.
What is UTC?
Coordinated Universal Time, the reference every time zone is measured from. It is in step with Greenwich Mean Time for everyday purposes.