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UTC

What is UTC?

Coordinated Universal Time — the one clock the rest are offsets from. It tracks atomic time, steered to stay close to the Earth’s spin. Every zone on this site is written as hours ahead of or behind UTC.

The zero the others are counted from

New York is UTC−5 in winter, UTC−4 in summer. Tokyo is UTC+9 all year. The number is the difference from this one clock.

This site solves the sky in UTC, then writes the result in your zone. That is why a full moon is one instant worldwide and a different clock reading in each city.

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Atomic time, steered to Earth

UTC is kept by atomic clocks and occasionally a leap second is added so it does not drift far from the Earth’s actual spin. The difference is tiny. For anything on this site, UTC is the honest now.

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Related questions

What is a time zone? A political stripe painted on a rotating ball. Noon is where the Sun is overhead. A zone is an argument about how far from that line a country is willing to sit, so clocks in that stripe all say the same hour.

What is the 24-hour clock? The clock that does not hide the afternoon. 07:00 is morning; 19:00 is evening. Science, airlines, hospitals and most of the world use it because 7:00 on a 12-hour clock throws away the answer to breakfast or dinner.

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Gravity, motion, time and space — one set of rules, seen from different sides: the glossary · the Sun, Earth & Moon · day & night on Earth · the solar system · the planets · gravity & orbits · sunrise & sunset · moon phases · time zones

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