Delta T, delta-T, deltaT, ΔT, or DT is the time difference obtained by subtracting Universal Time from Terrestrial Time. Universal Time (UT) is a time scale based on the Earth's rotation, which is somewhat irregular over short periods (days up to a century), thus any time based on it cannot have an accuracy better than 1 : 108. But the principal effect is over the long term: over many centuries tidal friction inexorably slows Earth's rate of rotation by about 2.3 ms/day/cy. However, there are other forces changing the rotation rate of the Earth. The most important one is believed to be a result of the melting of continental ice sheets at the end of the last ice age. This removed their tremendous weight, allowing the land under them to begin to rebound upward in the polar regions, which has