History & Leap Seconds

Standard time is younger than the railway. Before 1847 every town kept its own solar time; the telegraph and the timetable made that untenable. Here are the milestones, and the full record of leap seconds that keep UTC aligned with the spinning Earth.

Milestones in standard time

Year	Milestone
1847	British railways adopt Greenwich Mean Time — the first standard time in wide use.
1883	US and Canadian railroads adopt four standard continental time zones.
1884	The International Meridian Conference makes Greenwich the prime meridian.
1916	Germany introduces daylight saving time during WWI; others follow within months.
1972	UTC begins in its modern form, kept near UT1 by leap seconds.
2007	US extends DST by four weeks under the Energy Policy Act of 2005.
2011	Samoa skips 30 December entirely, jumping west of the date line.
2018	North Korea moves its clocks 30 minutes to realign with the South.
2022	Mexico abolishes DST for most of the country with weeks of notice.
2023	Lebanon delays its DST change, then reverses within days — two time zones for a weekend.
2035	Target date agreed at the 2022 General Conference on Weights and Measures for ending leap seconds.

Leap seconds

Leap seconds are announced by the IERS in Bulletin C about six months in advance. The 2022 General Conference on Weights and Measures resolved to stop inserting them by 2035.

27 leap seconds have been inserted since 1972, the last on 2016-12-31. Every one was positive.

#	Inserted at end of
1	1972-06-30
2	1972-12-31
3	1973-12-31
4	1974-12-31
5	1975-12-31
6	1976-12-31
7	1977-12-31
8	1978-12-31
9	1979-12-31
10	1981-06-30
11	1982-06-30
12	1983-06-30
13	1985-06-30
14	1987-12-31
15	1989-12-31
16	1990-12-31
17	1992-06-30
18	1993-06-30
19	1994-06-30
20	1995-12-31
21	1997-06-30
22	1998-12-31
23	2005-12-31
24	2008-12-31
25	2012-06-30
26	2015-06-30
27	2016-12-31