Greenwich Mean Time (GMT) is a time zone centered on the Greenwich meridian, which is the prime meridian used for international timekeeping. It is the same as Coordinated Universal Time (UTC). GMT is used in several countries around the world, including the United Kingdom, Ireland, Portugal, and Iceland.
GMT is not the same as other time zones, which are based on offsets from GMT. For example, the Eastern Standard Time (EST) zone in the United States is five hours behind GMT, while the Central European Time (CET) zone is one hour ahead of GMT.