| Royal Greenwich Observatory, London | The usual clock time is used in daily life and official civil time. Divides the world into 24 time zones based on the Prime Meridian in Greenwich, England. Adjusted by daylight saving time. |
| SI System | Globally accepted standard since 1967, using the second as the base unit of time. Coordinated at an international level, synchronized using precise atomic clocks. |
| International Atomic Time (TAI) | A weighted average of over 200 atomic clocks worldwide, synchronized using GPS signals and two-way satellite time transfers. Basis for Coordinated Universal Time (UTC) and Terrestrial Time (TT). |
| Universal Time (UT) | Based on mean solar time and the rotation of the Earth. Replaced Greenwich Mean Time (GMT) in 1928. Computed from precise astronomical observations, with variations like UT1 and UT2. |
| Coordinated Universal Time (UTC) | The primary time standard for civil timekeeping worldwide. Differs from UT1 by 0.9 seconds, adjusted with leap seconds. |
| Standard Time (ST) | The earliest internationally accepted time standard, was replaced by UT in 1928. Defined by the Royal Observatory in Greenwich, England. |
| Greenwich Mean Time (GMT) | The earliest internationally accepted time standard was replaced by UT in 1928. Defined by the Royal Observatory in Greenwich, England. |
| GPS Time | Used by the Global Positioning System (GPS), not corrected for Earth’s rotation. Differs from International Atomic Time by a constant offset. |
| Terrestrial Time (TT) | The site of the Prime Meridian used in Greenwich Mean Time (GMT), was established in 1675. |
| Sidereal Time | Used by astronomers to track stars based on Earth’s rotation relative to fixed stars, different from solar time. Varies due to Earth’s orbit. |
| System Time | Computer time is measured by a system clock, with variations like UNIX time, FILETIME, OpenVMS time, and RISC OS time. Coordinated using the Network Time Protocol (NTP). |
