As a noob, I recently went down the rabbit hole of time-setting for astronomy and space applications. It's an absolutely fascinating topic. Having the same tim

As a noob, I recently went down the rabbit hole of time-setting for astronomy and space applications. It's an absolutely fascinating topic. Having the same time in two places which are very distant apart, are in different inertial frames, or are in any combination of inertial and non-inertial, well, it's very difficult. The rhythms of life, and special and general relativity get in the way. But the need to convert from one time system to the other arises all the time. For example, if you want to to know if somebody looking from Newcastle, Australia, will be able to have direct line of sight to the asteroid 253 Mathilde on the night of the 23rd of September of 2177, then you need to work with UTC (which accounts for Earth changes in rotation speed which are unpredictable) and with coordinate frames which are inertial to the solar system (to apply Kepler laws). So you not only need to keep track of unpredictable things like Earth's rotation altering the precision of time, but also the drift caused on clocks by relativity. Note that problem here is that people live on Earth, and they want their time to reflect sunrises and sunsets, and they want it to track other natural cycles like the seasons. So we have UTC and UTC+n . Similarly, people living on the Moon may want to have a timesystem that reflects their sunraises and sunsets. But none of that matters to laws of physics that use a more fundamental notion of time, e.g. orbits. JPL has somewhere a set of routines that convert between different time-systems, and some software packages like Mathematica also come with the functionality. It does make sense to add a new timesystem for each body with strong enough gravity where people is going to live.