Leap Seconds History

Atomic clocks maintain Coordinated Universal Time (UTC) with extreme stability, whereas Earth's rotation (UT1) fluctuates due to tidal friction and core-mantle interactions. Leap seconds are inserted as needed to keep |UT1 - UTC| < 0.9 seconds.

System inception: 1 January 1972

Total leap seconds introduced: 28 positive leap seconds

Negative leap seconds: 0 (none ever introduced to date)

Source authority: International Earth Rotation and Reference Systems Service (IERS) via leap-seconds.list

Official Leap Second Chronology (28 Events)

Effective Date	NTP Timestamp	Cumulative TAI - UTC Offset
1 Jan 1972	`2272060800`	+10 seconds
1 Jul 1972	`2287785600`	+11 seconds
1 Jan 1973	`2303683200`	+12 seconds
1 Jan 1974	`2335219200`	+13 seconds
1 Jan 1975	`2366755200`	+14 seconds
1 Jan 1976	`2398291200`	+15 seconds
1 Jan 1977	`2429913600`	+16 seconds
1 Jan 1978	`2461449600`	+17 seconds
1 Jan 1979	`2492985600`	+18 seconds
1 Jan 1980	`2524521600`	+19 seconds
1 Jul 1981	`2571782400`	+20 seconds
1 Jul 1982	`2603318400`	+21 seconds
1 Jul 1983	`2634854400`	+22 seconds
1 Jul 1985	`2698012800`	+23 seconds
1 Jan 1988	`2776982400`	+24 seconds
1 Jan 1990	`2840140800`	+25 seconds
1 Jan 1991	`2871676800`	+26 seconds
1 Jul 1992	`2918937600`	+27 seconds
1 Jul 1993	`2950473600`	+28 seconds
1 Jul 1994	`2982009600`	+29 seconds
1 Jan 1996	`3029443200`	+30 seconds
1 Jul 1997	`3076704000`	+31 seconds
1 Jan 1999	`3124137600`	+32 seconds
1 Jan 2006	`3345062400`	+33 seconds
1 Jan 2009	`3439756800`	+34 seconds
1 Jul 2012	`3550089600`	+35 seconds
1 Jul 2015	`3644697600`	+36 seconds
1 Jan 2017	`3692217600`	+37 seconds