原子时
However, for the past several centuries, the length of the mean solar day has been increasing by about 1.4–1.7 ms per century, depending on the averaging time. By 1961, the mean solar day was already a millisecond or two longer than SI seconds. Therefore, time standards that change the date after precisely SI seconds, such as the International Atomic Time (TAI), would become increasingly ahead of time standards tied to the mean solar day, such as Universal Time (UT).