I'd like to try an experiment - building a world here, adding a new (hopefully interconnected) piece each week. I don't have a story in mind - that's sort of the point, seeing what emerges just from the creation of the world itself. At some point this is going to need a name, but that feels rather premature for the moment.

The sixty-fourth day of Fentebruary

 Our slightly-larger-than-earth ice planet rotates around its deadly start once every 120 years, while its three moons - a small dark, medium red and large white - loop around once every 426, 1150 and 5840 days respectively. We have four eclipses, occuring every 1380 days (dark-red), 7008 days (dark-white), 18396 days (red-white) and 22075 days (triple eclipse). That's earth-days, by the way - 24-hour rotations. We could change the day length (planetary rotation's more or less whatever you want it to be - it's dependant on how fast things were going when the planet was formed, and tidal locking stuff, so have at it. Anything up to about 96 hours is okay- after that, the temperature fluctutation between night and day gets too extreme. Also keep in mind the faster your rotation, the more volatile your weather.) but we've already got complicated things here, let's keep it simple.

We need to make a calendar. Well, we don't need to, but it helps later, especially with religious, spiritual or magical worldbuilding. It's also important to know how these people can measure time - the shortest 'month' they have, in our sense of the word, is longer than one of our years. How we measure time speaks volumes into our values , how our society works, what our myths and systems are, etc.

Many civilisations have several overlapping measurements that don't perfectly line up - consider our weeks, months and years. 365 does not evenly divide into 52, 7 or 12 - there are days left over that creep into the next year. So it's okay that our moons and eclipses don't all line up with our 120 years - it may be easier for you to manage as an author if many of them do, however - the reason I fudged things a little last week - having at least some of them line up makes things simpler.

The shortest period is the dark moon, which will probably represent their 'year' (there may be more appropriate yerms once other things are discovered/created). So far, it looks like we don't have much to mark within that year - but if we consider the phases of the moon, we can use those as the 'months' within our moon's year, to break the period down into smaller sections.

Months

A moon that has an orbit similar to the earth's moon will have eight phases, roughly equal in length. That makes each month roughly 53 days long, eight months to a year.

That is technically missing three days, though: 53*8=424, because 426 doesn't divide equally into eight. We have three options:

1. Use the 'leap year' system like our earth does with solar years
2. Create a people that will accept "53 and one quarter" days as the length of a month (maybe they're accountants)
3. Have 'days of the dead' or similar - special days that don't belong to any month to 'use up' the leftovers.

I'm going with option 3, both because it keeps things mathematically simpler, and I've always liked the idea of days-between-years. We have two, so there can be one extra in the new moon phase, and one in the full phase - putting both together would pull things out of sync, as the moon phases themselves aren't going to obey our special days.

Some things we should note, there - that month is about twice as long as our lunar month. From that, we can extrapolate a few details, like the length of a full moon - ours looks full (or new) for about three days, so the dark moon will look full for about five or six.

Weeks

We have months, now what about weeks? 53 days is a prime number, so our weeks aren't going to fit into our months, unless we have wacky weeks that change number as they continue. Not fitting in isn't really a problem, as we've already said - and plenty of numbers go into 426. A 6-day week gives us something roughly analogous to earth-time, and 71 weeks exactly in a year (again, I'm keeping things simple for my own sake.)

Calendars

At this point, I'd usually make a calendar, or the start of one - which I haven't pre-baked, unfortunately. However you like to represent time best, I usually go with a wheel, with concentric gear-like wheels for things that don't match up, like earthly months and weeks. In this case, we have seven rings, though only one has many actual divisions:

• The outermost solar-year ring of 120 years, marked in two places with the two triple equinoxes at 60 years. We can mark these in because they divide equally into the 120 - they'll happen at the same part of the wheel / year each time. This wheel will have to be huge in comparison, so the others can all sit inside it.
• The 50-year red-white eclipse, with one mark for the eclipse itself.
• The 19.2 year dark-white eclipse, with one mark for the eclipse.
• The white moon's 16-year "month", marked in eight places with its phases. None of its eclipses fall the same time within those 16 years, so we can't mark anything else on it.
• The 3.17-year dark-red eclipse, with one mark for the eclipse.
• The red moon's 3.15 year "month", also marked in eight places for its eight phases. Once again, the eclipses don't fall at the same time, so we can't mark them here.
• The dark moon's 426 day 'year', its eight phases/months (including the two 'days of the dead'), and its 71 weeks.

If we weren't dealing with such large numbers, we could replace the three eclipse rings with a 'meta' ring outside the solar-year ring that would mark each eclipse on it. In this case, such a ring would have to be 221225 years long, which isn't practical to build or look at, so we'll stick with concentric rings.

At some point I'll try to make this, to show you what on earth I'm talking about.

We'll come back to that calendar later when we're trying to make things like spiritual periods and things.