Time Zones Explained

A time zone is a region of the world that keeps the same standard time. The whole system exists for one simple reason: the Earth is a spinning sphere, so it is never the same time of day everywhere at once. When the Sun is overhead in Tokyo, it is the middle of the night in New York. Time zones let each part of the planet keep clocks that roughly match the position of the Sun overhead.

The prime meridian and the 24-hour idea

The Earth turns 360 degrees in 24 hours, which works out to 15 degrees of longitude per hour. That neat relationship is the foundation of the whole system. The starting line is the prime meridian, the line of 0 degrees longitude that runs through Greenwich in London. Local time at the prime meridian is the basis for UTC, the global reference clock.

Moving east of Greenwich, clocks run ahead of UTC; moving west, they run behind. Every 15 degrees of longitude corresponds, in principle, to one hour of difference. This gives roughly 24 zones circling the globe.

What a UTC offset means

Each zone is described by its offset from UTC — how many hours ahead or behind the reference it is. A few examples:

• London: UTC+0 in winter
• New York: UTC−5
• Tokyo: UTC+9
• India: UTC+5:30

To convert a time, you translate it to UTC and then apply the destination offset. Our time zone converter does exactly this, and it accounts for daylight saving automatically.

Why the map isn't tidy

If the world were purely mathematical, zone boundaries would be straight vertical lines every 15 degrees. In reality the map is jagged, because zones follow political borders instead. Countries want a single time nationwide for the sake of commerce and administration, so the lines bend around national and state boundaries.

This produces some striking results. China spans roughly five geographic zones but uses a single official time nationwide, so the Sun rises very late in the far west. India and a handful of other places use half-hour offsets like UTC+5:30, and Nepal famously uses a 45-minute offset (UTC+5:45). Altogether there are around 38 distinct offsets in use, not a clean 24.

Time zones and the date

Because zones stretch across a full 24 hours, at any given moment two different calendar dates exist somewhere on Earth. The boundary where the date flips is the International Date Line in the Pacific. It is why a flight can appear to arrive before it left, or land a whole day later than it took off.

Daylight saving on top

Many regions add a seasonal twist by shifting their offset for part of the year. This is daylight saving time, and it means a city's offset is not fixed — London is UTC+0 in January but UTC+1 in July. When you need to schedule across zones, it is safest to let a tool track all of this. Our meeting planner shows overlapping hours across several cities at once so you can find a slot that works for everyone.