ΔT

In precise timekeeping, ΔT (Delta T, delta-T, deltaT, or DT) is the time difference obtained by subtracting Universal Time (UT) from Terrestrial Time (TT): ΔT=TT−UT. The Earth's rotational speed is ν = (1/2π)dθ/dt, and a day corresponds to one period T = 1/ν. A rotational acceleration dν/dt gives a rate of change of the period of dT/dt = −(dν/dt)/ν, which is usually expressed as α = ν dT/dt = −(dν/dt)/ν. This has units of 1/time, and is commonly quoted as ms/day/cy.