GMT is more properly called Universal Time at present, and is calculated from observations of extra-galactic radio sources, and then converted into several forms, including UT0 (UT at the remote observatory), UT1 (UT corrected for polar motion), and UTC (UT in discrete SI seconds within 0.9 s of UT1).
Od tedaj naprej so GMT izračunavali prek opazovanj na drugih dejavnih observatorijih.