In astronomical time scales, the difference between Terrestrial Time (TT) and Universal Time (UT). Currently this difference is nearly one minute and is increasing at the rate of about 2 s every three years. It arises because the Earth’s rate of axial rotation is not precisely uniform, but shows certain irregularities and is gradually slowing down. Universal Time is tied to the rotation of the Earth through its link with sidereal time, while Terrestrial Time is defined by the orbital dynamics of bodies in the Solar System. Ancient Chinese and Babylonian eclipse records show that delta T was −5 hours in the year −500 and decreased rapidly to less than +1800 seconds by the year 1300. It reached a minimum in the late nineteenth century. Due to the constant tidal deceleration caused by the Moon, it is expected to increase rapidly in future centuries.

http://asa.hmnao.com/SecK/DeltaT.html Values of delta T for past, present, and future.