ires revision periodically.
So, a parameterised universal leap year calculation that minimises the drift of days in a year, makes sense. The only parameter required is the length of the year in local days as a real number. The start of Era may or may be a leap year itself.
In summary, a calendar is the synchronisation of 2 cycles by inserting 'leap' periods. On other planets a leap second would be required too every so often, given the second is a fraction of an Earth day, and it is sensible to keep the SI units.
Structure of dates
A calendar minimally requires 2 numbers to make it work, the number of planetary rotations in the orbit (the Earth around the Sun for example). In essence, we are trying to maximise the usefulness and predictive capability of the model without making it too complicated (and keeping the number reasonably small and tractable). So we could simply say a date is Year.Day (with 365.24219878 days in a year approximately on Earth). To turn this into 2 integers we require an day to be added at the end of the year periodically, called a leap year. If it were not added at the end of the year the day numbers would jump about between normal and leap years, which is highly undesirable.
Now, traditionally on earth we have tried to accommodate lunar cycles into this scheme too, calling them months. This does not 'fit' very well mathematically. However, as humans we find the additional concepts of months and weeks convenient for our activities. To justify this we think of months as approximating lunar cycles (but the drift makes this rather impractical) or with reference to the appearance of the 'fixed' stars, on which our brains project patterns, a sequence of constellations as they are called. Historically (in the West), this has resulted in a calendar with 3 cycles, years, months, and days (or 4 cycles with weeks). We are so familiar with this convention, it seems 'natural', it is actually just locally convenient and historical.
With the introduced concepts of months and weeks, and thousands of years 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 i