s of tradition, we like to name these concepts and number them from one. This works reasonably well on Earth, however, if we are to keep 28 day months on other planets then the number of months required becomes less manageable as we get further from the sun. On Uranus this would require approximately 741.298, 28 (local) day months which is inconvenient. Even on Jupiter this would be 154.627 months in a year. Mars is not too bad at 23.878 months per year. Week days, if kept to 7 days could retain the current names, but it might be desirable to use culturally and historically neutral names.
Recommendation, for consistency number these concepts (month days and week days). For ease of conversion from calendar to calendar, start the numbering at zero. A competition for additional naming of months/week days can be held when a new planet is colonised if sensible, but humans on other planets need not know your local naming conventions (just the year and day length) to be able to communicate and convert dates if the names are underpinned by numbers.
Where will we live
Now with reference to the other planets, the outer planets are gas giants, so we are unlikely to live on the surface of these planets. We may well live on the satellites though. So, we do not necessarily need to use the day length of the planet as the day length we adopt. We will not be interested in agriculture on the surface of the satellite, as we will live in artificial environments. So the day length can be either local to the satellite (if tolerable for humans) or we could simply adopt Earths day length and ignore light/dark period duration (in so much as that matters given the sun is distant).
For the Earths Moon, a 'local' day is about 2 weeks, so ignoring local days and adopting Earths day length makes sense, indeed, just adopting Earths solar calendar would be sensible. On Mars, the local day length is 24.65979 hours, so we could adopt the local day and year and have a completely local Martian calendar that would be tolerable for humans. The year is approximately 668.591 local days long so there would be a lot of leap years, but this does not really matter.
For artificial space stations, adopting the calendar of the planet being orbited makes sense. For everyday usage (around say Jupiter), using the Earth calendar internally would be convenient (especially given the days are so short).
So, in conclusion there are some choices about what is convenient for humans to adopt as a convention in new places to live. A generic model can be parameterised to take into account these variants. The only fundamental concepts for a calendar are the longer period (for example the year), and the shorter period (the day). The most basic, but complete calendar is then Year+Day. Other formats to include months and weeks can then be accommodated on top of this, again with automated conversion between formats.
Include time of day
When the length of a day varies between planets converting date/times between the two planets means that the time of day will change. For example 10:00 on Earth (assume UTC) would be some other time on Mars, assume conveniently it is also 10:00 there. A meeting is held (ignore light delay inconvenience). We want to have another meeting the following week. On Earth that time will again be 10:00 (assuming Earth time is 'dominant'), but as a Martian day is 0.65979 hours longer we need to subtract that from the meeting time, so 7 x 0.65979 x (24 / 24.65979) = 4.494958. The Martian time for the meeting would be approximately 05:30 in the morning. Similarly, if the Martian time were 'dominant' then the earth time would be later in the day. When scheduling monthly, this would push the meeting into the 'previous' day on Mars. So, it is inaccurate to model just the calendar, the relative length of a day must also be taken into account. There are other confounding factors. Such as timezones, daylight savings and the relative light delay between planets. For this reason, it makes sense to reduce the number of variables. We can do nothing about relative light delay