GMT/UTC has a British imperial past (i.e., the time at Greenwich) and was colonialistically imposed on the world. Many countries seem to prefer to measure time with respect to themselves. I wish we could think of something that all of humanity shares, and measure universal time with respect to that.
Time (e.g., seconds and multiples thereof) since the Big Bang, or some other large astronomical event, is perhaps the most universal and future-proof idea. Big Bang time is not yet possible, because humanity’s best astronomy instruments currently can measure when the Big Bang happened to a precision of only about ±21 million years. Whenever it becomes possible, perhaps only the last few digits could be used, for convenience.
The time at the center of the Earth, or at some location that is administered by an open-member international committee, is another idea. The poles probably wouldn’t work, because the sun rises and sets there only once per year. Time runs slightly slower at the center of the Earth than at the surface, due to Earth’s gravity well and time dilation, but I have read it is only about one year slower for every billion years. I understand that GMT/UTC is local mean solar time, meaning the apparent position of the sun relative to a fixed point on a sphere. Even if the sphere was much smaller, one still would have to pick a point on it.
Can you think of anything else that all of humanity (going into the future) shares with respect to which we could measure time?