Temporal Reference System Terrestrial Time https://www.commoncoreontologies.org/InformationEntityOntology TT TT is an astronomical time standard that is a theoretical ideal designed to be free of the irregularities in Earth's rotation and is primarily used for time measurements of astronomical observations made from the surface of Earth. It was formerly called Terrestrial Dynamical Time (TDT), which was formulated to replace Ephemeris Time (ET).