system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why was Greenwich Mean Time changed to Coordinated Universal Time?
gpt
It wasn't. GMT was changed to UT1. 

GMT and UTC are actually completely different: GMT is time based on the *sun*. "Mean" in GMT refers to the fact that it is based on *mean* (i.e. average) solar time. On the other hand, UTC is *atomic* time, based on vibrations of radiation given off by atoms. At the precision of milliseconds, the Solar day is variable in length, Earth's rotation is slowing, and a solar day is no longer exactly equal to 86,400 seconds. These clocks therefore tick at different rates.

But it makes sense that atomic time should roughly agree (i.e., be "coordinated") with solar time for practical purposes. That is why, every once in a while, a leap second is added to UTC: to keep it within 1 second of GMT (or really, GMT's successor, UT1).

TL;DR: GMT (or really, UT1 now) is solar time, whose rate of flow is slower now than in the past, and varies at the level of milliseconds because of changes in the Earth's rotation rate. UTC is atomic time, with an absolutely constant rate of flow that must be periodically adjusted ("coordinated") to stay within 1 second of UT1.