Greenwich sidereal date
Greenwich sidereal date(GSD) A concept analogous to that of the Julian date. It is the number of sidereal days that have elapsed at Greenwich since the beginning of the sidereal day that was in progress on Julian date 0.0. The integral part of the date is the Greenwich sidereal day number; the fractional part is the Greenwich sidereal time.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006