I see you are a multi directional artist, drawing anime style art style and Sci-Fi style as well
But regarding planets. It is basic knowledge that one day is the rotation period of a planet around it's axis, and a year is the rotation around it's sun (or mother planet?). But how you call a moon's rotation around it's mother planet? For example the Luna (our Moon) because of it's slow rotation speed, one moon day is equivalent roughly one Earth month while it's year is the same as well as it needs this one moth to rotate around us.
But let's say there is an Earth type planet which rotates around a Gas Giant in 100 days, while the Gas giant rotates around the Sun in 4 years. How would such planet use time? Short and Long Years?
The orbit of a planet around a star is called a year, and the orbit of a moon around a planet is called a month. Luna is unusual in that it is tidelocked to Earth (the same face always points to Earth), so Luna's month is the same length as its day.
In your example, the Earth type planet is a moon, so you could say that its month is 100 Earth days and its year is 4 Earth years. The planet's seasons won't change significantly as it orbits the gas giant, as it will still have the same orientation towards the sun.