UTC (備忘)

LOD = length of day
TAI = International Atomic Time
UTC = Coordinated Universal Time
GMT = Greenwich Mean Time (basically UT1)

rotational speed of the earth is not uniform.
LOD fluctuates.

rotational speed is also slowing down on average.
LOD increases at a rate of about 1.7 ms/cent.

LOD = 86400 s (GMT, by definition)
= 86400 s (TAI in 1820)
= 86400.002 s (TAI at the end of 20th cent)
= 86400.004 s (TAI at the end of 21th cent)

1 day (GMT) is defined to be LOD.
1 day (TAI) is defined to be 86400 s (TAI).
LOD = 1 day (GMT) =/= 1 day (TAI), except in 1820

timestamp:
UTC = TAI with correction = a close approx. of GMT (+/-1s)
1 s (UTC) = 1 s (TAI) by definition
correction = leap second at irregular intervals
(+1s/500day at the end of 20th cent, on avaerage)
(+1s/250day at the end of 21th cent, on avaerage)

leap second announced:

wikipedia