How time zones work

When it is noon in Delhi, it is early morning in London and the middle of the night in New York. Time zones are the agreed way of handling this.

The Earth turns

The Earth makes one full turn, 360 degrees, in 24 hours. That is 15 degrees every hour. Noon, the moment the Sun is highest, sweeps westward around the globe at that speed. If every town set its clock by its own noon, clocks would differ by a few minutes from one town to the next. That is how it was until the 1800s.

Railways needed one time

Trains and the telegraph made local times a problem, because a timetable needs one clock. Countries began to adopt a single standard time. In 1884 a conference in Washington chose the line through Greenwich, in London, as the zero line of longitude, and the system of zones counted from Greenwich grew from there.

The idea: 24 zones of one hour

In the simple model, the world is cut into 24 strips, each 15 degrees wide. Each strip keeps one time, and each is one hour ahead of the strip to its west.

The reality: countries decide

Zone borders are political. Each country chooses its own time. That is why the lines on a time zone map zigzag along borders, why China uses one time for a land five strips wide, and why India, Nepal and a few others use half or quarter hours to match their own central longitude.

UTC, the reference

All zones are written as an offset from Coordinated Universal Time, UTC. UTC+5:30 means five and a half hours ahead of UTC. UTC itself is kept by atomic clocks and has no summer time.

The date line

Travel far enough east, adding hours, and you must at some point jump back a whole day. That happens at the International Date Line in the Pacific Ocean, on the opposite side of the world from Greenwich. Cross it westward and the date moves one day forward.

Try it

Open the list of UTC offsets to see all of them with the time right now, or convert a time.