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Universal Time Coordinated. Timekeeping system that relies on atomic clocks to provide accurate measurements of the second, while remaining coordinated with the Earth’s rotation, which is much more irregular. To stay synchronised, UTC has to be adjusted every so often by adding one second to the day, called a leap second, usually between June 30 and July 1, or between December 31 and January 1. This is achieved by counting 23h59’59”, 23h59’60” then 00h00’00”. This correction means that the Sun is always at its zenith at noon exactly (accurate to the second)