Ok, this is take two. Hopefully this is specific enough. I have a link to the original question here.
And for those of you eager to see my sources for this, I'll link two videos here from the YouTube Channel Artifexian.
Worldbuilding: Terresstrial Moons
Worldbuilding: Gas Giant Moon Systems and Habitable Moons
I have a 6-planet solar system (only three planets have been fleshed out so far), with a purple gas giant in the Habitable zone. It has 3 Earth-ish sized moons: one small ice-covered ocean moon (like Europa), one swampy rainforest moon (like Endor/Dagobah), and one Desert moon with 60% of its surface covered in dry land. The desert moon's 'year' is 147 Earth days long, but due to the moon's rotation, it's actually more like 117 days long.
(I'll put stats and numbers in comments, if any of you wanna recreate it in Universe Sandbox or stuff then let me know.)
An intelligent species will inhabit the desert moon, the furthest one out. My big question is what are some possible ways they can create a calendar?
I've considered making the year based around the moon itself, but is it possible that they could see that their planet (the big, bright purple thing in the sky) orbits the sun (the big REALLY bright thing in the sky)? Or maybe they can tie time periods to the other two big moons?
(P.S. The moon orbits at a sufficient angle and distance that regular eclipses aren't a thing, so if anything, solar and lunar eclipses would be about as regular as Earth's eclipses.)
Edit: here's the planet and moons' mechanics, per request. Units are given in terms of Earth. (Except for the planet, whose mass and radius is given based on Jupiter.)
Planet 2: Purple Habitable Gas Giant
Dist: 1.2 AU
M: 2.15 Mj
R: 2.89 Rj
g: ??? m/s^2
d: 0.88 g/cm^3
a: 1.2 AU
Year: 1.3 Earth-Years
Moons: 7 (3 Earth-Sized)
Moon 1: Ice World
M: 0.74 Me
R: 1.01 Re
g: 6.93 m/s^2
d: 3.85 g/cm^3
a: 2,000,000 km (0.01336 AU)
b: ??? km (0.0 AU)
Year: 0.57 days (12.5 Earth Days)
It's icy because it formed with the gas giant out in the distance. It lacks any form of greenhouse gasses, and the surface ice reflects sunlight away, keeping it from melting fast enough. Tidal forces from the planet, though, keep it warm internally, enough to support a subterranean ocean.
Moon 2: Endagobah (Swamp World)
M: 0.76 Me
R: 0.92 Re
g: 8.85 m/s^2
d: 5.40 g/cm^3
a: 5,161,126 km (0.0345 AU)
b: 5,410,000 km (0.03 AU)
Year: 42 days (55.4 Earth Days)
This place has bugs. Nobody'll go there anytime soon.
Moon 3: Desert World
M: 1.36 Me
R: 1.103 Re
g: 10.99 m/s^2
d: 5.59 g/cm^3
a: 10,500,000 km (0.0702 AU)
b: 10,499,112 km (0.0718 AU)
Periapsis: 10,363,500 km (0.069 AU)
Apoapsis: 10,636,500 km (0.0711 AU)
Year: 117.6 Days (147 Earth Days)