There has been 24 leap seconds added in the 37 years that this method of coordinating time has been in use. International Atomic Time (TAI) is now 34 seconds fast of UTC. The earth rotates (with respect to the mean sun) a little slower than one revolution per 86400 seconds. The length of a day varies from about 86400.001 to 86400.003 seconds per day and does not have long term stability.
This is a note I wrote (in California) the morning of 2005-12-31: At 3:59:60 PM PST this afternoon (2005-12-31 23:59:60 UTC) a leap second is going to be added to Coordinated Universal Time (UTC). UTC, adjusted for our time zone, is what we set our clocks to. If you have a radio that can receive WWV (5, 10, 15, 20, 25, 30 MHz) you can listen to when the leap second happens. I will be listening at home. The leap second will last for one second from 2005- 12-31 23:59:60 UTC to 2006-01-01 00:00:00 UTC.
See the plot of leap seconds. Actually it is two plots in one. We have UTC - TAI as a function of time and UT1 - TAI as a function of time. UTC is adjusted in one-second steps to closely approximate UT1.
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