Also, in terms of age, we could say that people in Arum mature faster than us, so that an 18 year old in Arum is equivalent to an 18 year old in real life, in terms of how mature they are. Let's see how other people feel about that.
I came up with a calendar for my game a while back. It is 364 days long, with 13 months 28 days (with a leap day every 10 years) and is based on a sidereal zodiac, with a simple birth-sign system similar to the one in the Classical Zodiac. I have left the months unnamed.
The reason we have twelve months is because the Sumerians had a thing for the number 12. I would be fine with twelve months of thirty days, but a calendar based on thirteen lunar months of twenty-eight days would be cool, too. I'm assuming a single moon similar to our own.
I don't anticipate the need for a calendar in my game, so I feel odd about naming the months. I think true to the Arum project, this kind of decision should be made by someone who includes it in a completed game, and then the calendar will be canon for that region in that era.
I think, a new calendar per se is a good idea. Though I also have some trouble with only 222 day a year. Also we will develop and play the games mostly in the medivial and renaissance age. On earth at that time every continent had its own calendar. Think of the Maya for example. Therefore your idea with a calendar is good and I guess can be used for a certain game. But it is nowhere near canon for the whole world. Even on the big northwestern continent could exists two or more calendars because of the big mountains in the middle, which serve as hurdle for travel, trade and news.
A universal, canon calendar for the whole world is a bad idea, since cultures perceive time differently, depending on their location (seasons), the level of their knowledge (astronomy, math - this relates to the accuracy of the calendar), it's even influenced by their religious beliefs and lifestyle (a nomadic tribe of hunters doesn't really have a need for a calendar).
This means that unless Arum is a world populated by a single, dominant culture/race, or a globalized plane, universal calendar is a failure.
Still, this is an interesting concept for a specific game/culture/race/civilization.