“A day, as we all know, is 24 hours, also known as 1,440 minutes or 86,400 seconds. Or if you’re an atomic clock maker, exactly 794,243,384,928,000 cycles of microwave radiation tuned to the ticking rate of the cesium atom. Regardless, a day is a day, right?
Actually, no: An Earth day isn’t a fixed interval of time. You may have noticed news stories reporting that Earth has shaved a millisecond (thousandth of a second) or more off the time it takes to make a full rotation, or conversely, that the planet’s rotation has slowed down slightly. While some of these stories exaggerate the impacts of these changes, their basic premise is correct: Earth’s seemingly steady rotation can, at times, seem as fickle as the weather.
This wouldn’t be a problem — or even noticeable — except that we have two different ways of telling time that don’t quite agree with each other. The one that probably feels most intuitive is solar time, in which a day is defined by the time it takes our planet Earth to rotate once on its axis with respect to the Sun. Scientists track solar time using observations of very distant, very bright objects called quasars. The day measured in this way is called the sidereal day.
But the official modern definition of time, as agreed on by the international scientific community, doesn’t refer to the planet or stars at all. “
https://www.nist.gov/time-frequency/what-determines-length-day