When humanity began establishing multiple colonies and making contact with species existing across different planets, there quickly became a need to coordinate some sort of time keeping system everyone could agree with.
On Earth, a time known as Coordinated Universal Time, or UTC, was kept. All time zones on the planet were designated relative to this universal time, simply adding or subtracting some amount of hours. This same time standard is now designated as Standard Earth Time, or SET.
Complexities arise, however, since most planets do not have the same length of day as Earth, meaning simple addition and subtraction is not sufficient. Local time coordinators for each planet update yearly time conversion tables to convert their local Coordinated Planetary Time (analogous to the former UTC) to SET. Then, of course, planetary time zones must be considered, adding or subtracting from the PTC.
Years are also considered relative to Earth standard, making the value of a year somewhat arbitrary. In one standard galactic year, a planet may go through a winter, spring, summer, fall, and another winter and spring; then, next year, the planet will go through a summer, fall, winter, spring, and another summer and fall.
Thus, to keep logistically consistent dates for the time of the planet’s rotational cycle, local calendars are also used. These are important for coordinating things like growing seasons and holidays, e.g., the planet Deisos celebrates fishing day every Mercedonius 1st, even though this local date occurs two to three times within a galactic standard year. Deisos has short years and long days, meaning that their local date contains fewer months than the standard 12.
For business, SET date and time is normally used, as well as for purposes such as announcing release dates. However, advertisements will usually be converted to local dates. Messaging applications will also typically automatically convert to your local date from SET or another local date, if desired.
Nova Satus, notably, follows SET completely. Days are approximately the same length (with small accumulating errors being accounted for by an occasional leap day), and while years are longer, seasons on the planet are not particularly strong due to its lower axial tilt. Its colonized moon Revenite follows the same time as well, despite having much shorter days. Its citizens are used to being out in the dark or sleeping when it’s light out.