Greenwich Mean Time (GMT) was used as the global standard for the reference time. Greenwich is a London district through which the prime meridian passes (longitude 0° 0' 0"), which forms the reference for Greenwich Mean Time.
From the zero longitude in Greenwich, the globe is divided into west and east longitudes, each by 180°.
GMT time is the time that prevails at Greenwich Observatory, England. It is the time reference for astronomical time and is also referred to as UT1, which stands for Universal Time 1. The GMT time scale UT1 and the Coordinated Universal Time( UTC) differ slightly, which is compensated by the inserted leap seconds. Greenwich Mean Time has been replaced by Universal Time Coordinated (UTC), the coordinated world time.