TAI (weighted average of atomic clocks, correcting for relativistic effects due to altitude) is the basis for UTC, and is currently 35 seconds ahead of UTC. Pr

TAI (weighted average of atomic clocks, correcting for relativistic effects due to altitude) is the basis for UTC, and is currently 35 seconds ahead of UTC. Prior to 1972, UTC and TAI used different definitions of a second (UTC used a definition based on positions of celestial bodies while TAI used a definition based on the hyperfine transition of cesium, and the two were periodically manually "stepped" back into sync). As of 1972 they use the same definition of the second (TAI's definition, which is also the SI definition), and a final "step" was applied to cause the difference in the two times to be an integer number of TAI seconds. That created an offset of 10 seconds, which has since grown with the addition of 25 (and soon to be 26) positive and zero negative leap seconds to UTC.