The UK, Portugal, Greenland, Iceland and several West African countries use Greenwich Mean Time, sometimes called Western European Time. GMT is also known as 'Zulu Time' in military slang.
International organizations consider GMT the standard time zone. GMT-0 is GMT without any amount added or subtracted. Other time zones are defined in their relation to GMT, such as GMT-3, or three hours before GMT, for Atlantic Daylight Time, used primarily in Halifax in eastern Canada. In the opposite direction, GMT+3 is in the Middle East and eastern Africa and maintains a time three hours past GMT.
Greenwich Mean Time is similar but not identical to Coordinated Universal Time, commonly known as UTC, due to a disagreement and subsequent compromise between Anglophone and Francophone countries over the acronym. The distinction is minor, down to about a second's difference in accuracy in UTC's favor. This makes UTC the preferred choice in scientific work, particularly concerning space independent of Earth's rotation.
GMT is based on the rotation of the earth, while UTC measures the decay of radioactive material. The rotation of the Earth is slightly irregular, while atomic decay is unfailingly constant. This means that UTC requires occasional adjustments known as 'leap seconds' to stay aligned with the Earth's current position.