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.