1 Answer

Based on the Greenwich mean time which is the absolute reference.

Greenwich Mean Time (GMT) was established in 1884 at the International Meridian Conference, when it was decided to the place the Prime Meridian at Greenwich, England. And Why Greenwich?Per the web World Time Zones.

GMT is the mean solar time at the prime meridian (the line of longitude where east meets west), which runs through Greenwich in the UK. GMT is measured from midnight, so the new day begins at 0000 GMT and the time six hours later would be expressed as 0600 GMT. "Mean solar time" refers to the local time on the prime meridian, using the hypothetical rather than the actual position of the sun. The earth does not orbit the sun evenly, and its axis is tilted in relation to the sun. In the course of one solar day, the earth spins once on its axis and it takes 365 days to complete one full revolution of the sun. However, the relative position of the sun varies throughout the year. At particular times it apparently slows down or speeds up - shortening and lengthening the day by up to 16 minutes. That's why we need to use the average (mean) day length and a hypothetical mean sun when calculating GMT.

Prior to 1925, GMT for sailors and astronomers was measured from midday, while the "civil day" began at midnight. This civil time was also known as GMT, which made for much confusion! In 1925 it was agreed that the two GMTs should be synchronised. Time was thereafter measured from midnight to midnight.