What is time measured according to?it is supposed be a three letter word and ends with a "t".
I think the term you're probably looking for is GMT, which stands for Greenwich Mean Time. This refers to the time at Greenwich, England (which is now a part of London). All the time zones on earth are talked about with reference to GMT. So you'll see the time in some place on earth referred to as "GMT minus 8" which is the time zone I'm in. This means that when it's 7 AM where I am, it's 3 PM in Greenwich.
This is also called Coordinated Universal Time, but that's abbreviated as UTC so it wouldn't work given your clue.
You make puzzle when you the answer is 3 lettered word ends with t. The main reference of time is the sun. For a common man the time is a day from sun rise to next sun rise. From midday to next midday. From any angular postion of the sun to the next similar position. Our common sense of time is sun related.
But the actual day is the one full rotational period of earth, called a solar day. The reference for the rotation may be sun or star. Stars are more fixed as they are more distant and the apprent postion of the sun is not fixed due eccentricity of the orbit of earth and the nearness of the sun compared to stars.
A mean solar day is taken to be 864000 seconds. The second is the SI unit of time in science.