Greenwich Mean Time (GMT), the name for mean solar time of the longitude (0∘) of the Royal Greenwich Observatory in England. The meridian at this longitude is called the prime meridian or Greenwich meridian. Greenwich Mean Time (GMT) has been used to clearly designate epoch by avoiding confusing references to local time systems (zones). Since Cairo is 30∘E longitude, one hour is 15 degree longitude; therefore Cairo is two hours ahead of London.