Leap second

Epic card · No. 3672

Leap seconds or intercalary seconds were one-second adjustments occasionally applied to Coordinated Universal Time (UTC) between 1972 and 2016, to accommodate the difference between International Atomic Time (TAI), as measured precisely by atomic clocks, and observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently runs ahead of observed solar time unless it is reset to UT1 as needed: the standard set a maximum permitted variance of ±0.9 seconds. The leap-second facility existed to provide this adjustment when the variance threshold was reached. The leap second was introduced in 1972. At that time, UTC already lagged behind TAI by 10 seconds because of earlier methods of managing the difference. In total, 27 leap seconds were added to UTC, with the final occurring on 31 December 2016; thus the...

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