Coordinated Universal Time (UTC) is the scale used to compare local times. A clock in Tokyo, London, or New York is an offset from UTC, written as UTC+9 or UTC-5. The offset can change when daylight saving starts or ends.
Greenwich Mean Time (GMT) is the civil time kept in the United Kingdom and Ireland in winter. For setting a meeting or reading a timetable, GMT and UTC name the same hour. The difference shows up only when a leap second is inserted into UTC, which does not happen every year.
This site reads local civil time from the IANA time zone database. The database carries the offsets and the dates governments have set for clock changes. It is not a political map.