Define Delta T as a change in time if you are solving a physics or math problem. For example, the acceleration of a body, A, will equal the change in velocity, Delta V, divided by the change in time, Delta T -- so Delta T will be the end time minus the start time. Make sure you have the correct units.
Define Delta T as a change in temperature when you are solving a problem in thermodynamics. If you are calculating specific heat, the heat capacity, c, will equal the amount of thermal energy a body can absorb -- Delta Q, divided by the change in temperature, Delta T. Also apply Delta T to atmospheric temperatures in meteorology.
Define Delta T as the difference between Universal Time and Terrestrial Time if you are referring to the astronomical calculation of time. Universal Time -- also known as UT or Greenwich Mean Time -- is a time scale that is based on the rotation of the Earth. However, due to gravitational influences, the Earth does not rotate at a perfectly constant rate. Terrestrial Time is a theoretical time scale, a kind of atomic clock, that imagines Earth rotating at a perfectly constant rate. Think of it as an ideal time scale.