The Greenwich Mean Time serves as the base reference used for time zones around the world and is based on the line of longitude that passes through the Greenwich district of London. A hypothetical average day in Greenwich provides the basis of the GMT time zone.
Time zones based on the GMT are typically depicted with either a minus or plus hours value after the GMT characters. GMT measurements do not take into account the interaction between the earth and the sun, which can cause fluctuations in time. Consequently, the UTC, based on the atomic clock, has replaced GMT. The UTC occasionally adds leap seconds to the clock, much as a day is added every leap year.