In astronomy, what matters most is a continuous count free of irregularities such as leap years, months of varying lengths, or time zones.
Consequently, various time scales have been introduced:
| UTC | Universal Time coordinated | Coordinated Universal Time (English: Coordinated Universal Time; French: Temps universel coordonné) is the current universal time system, introduced in 1972, which is also known as Zulu time. A complete UTC time notation consists of the time and the calendar date. For national or regional time zones, the difference in hours from UTC is usually indicated. |
| UT | Universal Time | Universal Time (UT) is the mean solar time of the prime meridian passing through the Greenwich Observatory, as determined by astronomical observation. It is a universal time scale that reflects the measured rotation of the Earth. In the geosciences and astrometry, it is the most widely used time system. The variant UT1 is directly related to the phase angle of the Earth’s rotation and serves as the reference time for Coordinated Universal Time (UTC), which was introduced in 1972. This time scale—which, as atomic time, is independent of the Earth’s rotation and completely uniform—is adjusted to UT1 as needed by adding occasional leap seconds, thus constituting the contribution of astronomical chronology to the precise determination of time. |
| TT | Terrestrical Time | Terrestrial Time is a modern astronomical time standard defined by the International Astronomical Union and used primarily for time measurements in astronomical observations made from the Earth’s surface. |
The Julian Date (JD) is used for counting: It specifies the time, in days and fractions of a day, that has elapsed since 12:00 UT on January 1, −4712 (4713 B.C.).
Due to different applications, various variants have been introduced:
| MJD | Modified Julian Date | JD − 2.400.000,5 |
| JD Dublin | Julian Date Dublin | Die Dublin-Day count is a shortend Julian Date starting 12:00 Noon at 31st December 1899. |
| JD CNES | Julian Date CNES | |
| JD CCSDS | Julian Date CCSDS | |
| MJD2000 | Modified Julian Date 2000 |