It is similar to UTC, however there are some small differences between the two. Including that the Greenwich uses a straight line to signal the Time Zone, while in the UTC you can put which time zone you want, even if it does not need this time zone. One example is China, which uses one same time zone, meaning that in some places like Beijing 12:00 PM can be 3:00PM on the western part of the country.
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Greenwich Mean Time Facts for Kids. Kiddle Encyclopedia.