Systems of Time Keeping

What do you mean, the appointement was in Martian time?

— Leea Nikula, stressed businesswoman

Even back in the 20th century, time keeping was known to be a bit of a nightmare to those who had to deal with it on a large scale, thanks to issues such as time zones. For a brief time in history matters seemed to improve as one coutry after another started to get rid of additional complications such as daylight savings time. Then, things got much worse, as humanity became an interplanetary civilization. As it turns out, different places experiencing different day and night cycles is an order of magnitude more complicated than them simply being at different phases of the same cycle.

UTC

Formally, dates and times are defined relative to coordinated universal time (UTC). Any local time is converted to it when it needs to be communicated to someone with a different local time. UTC is also used in places that don't have a day-night cycle, or where it differs too much from the roughly 24 hour cycle on which humans function. This includes spacecraft and space stations and also some bodies such as Mercury. There are exceptions of course, as some such places choose to instead operate on the time of a planet they wish to keep in sync with.

Local Time

For people living on a planet, it is more convenient to use a system syncronized with the planets rotation, so that a certain hour corresponds to a certain time of day. Since no two planets rotate at the same speed, every inhabited planet has its own system. Time is still measured in normal hours, however the day length differs from Earth's 24 hours. In general this is not an integer number of hours, of course. On Mars, for example, it is 24 hours and 37 minutes. This "incomplete" hour of the day is called a time slip. Depending on the planet, it is either placed at midnight or noon. It often is of special significance in the local culture. The length of a year also varies. To keep track of the seasons on a planet, there are also local calendars running in parallel to the normal Earth calendar.

Interstellar Ships

On interstellar spacecraft, things get extra weird. Moving at relativistic speed kind of ruins the whole concept of simultaneity. It is pointless for the people aboard such a ship to try to stay in sync with some planet. Instead, the ships maintain their own 24 hour cycle. A calendar is replaced by a countdown to arrival in the ship's time.