Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich in Lonon. It is now often used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, although strictly UTC is an atomic time scale which only approximates GMT in the old sense. It is also used to refer to Universal Time (UT), which is the astronomical concept that directly replaced the original GMT. In the community of Greenwich, GMT (in the form of UTC) is the official time only during winter (during summer the time in Greenwich is British Summer Time rather than GMT).