The concept of time that is used as the variable in gravitational equations of motion. The dynamical time formerly used for the computation of ephemerides was ephemeris time ET, but this has been replaced by terrestrial dynamical time TDT and barycentric dynamical time TDB. TDT is essentially a continuation of ET, intended for use as the timescale in calculations of geocentric ephemerides. No assumption is made about the theory of gravity used. It is based on the SI second and measured by atomic clocks. It commenced in 1977, when the relationship between TDT and International Atomic Time TAI was defined to be: 1977 Jan 1, 0 hours TAI = 1977 Jan 1.000 372 5 TDT. TDB is intended for use in the equations of motion of planetary bodies referred to the barycentre of the solar system. It is not uniquely defined but depends on the theory of gravitation that is adopted. However, it is stipulated that it differs from TDT by only periodic discrepancies.