• Any device for keeping track of the passage of dates. Most calendars are astronomical in basis, depending on the Sun and Moon. But they all suffer from the problem that the day, month and year are slightly variable and do not represent simple fractions or multiples of each other. Much ingenuity has been exercised on this problem over many millennia, giving rise, e.g., to the Gregorian calendar now in use in most of the world, which adds a leap day when the year number is divisible by four, except in a century year, which has a leap day only if divisible by 400. This system is accurate to one day in over 3000 years. Increased accuracy in measurement brings new complications. Time is now measured by atomic clocks, which are more regular than the Earth’s rotation, which is gradually slowing. This necessitates the insertion of ‘leap seconds.’

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