Sounds like they just ignore any calendar weirdness. I don't envy anyone the task of trying to properly program this. You basically have to decide for everything time related in the simulation whether it is tied to the calendar or the physical passage of time. Sometimes you would have to split variables, like characters are legally older, but no closer to old age. Bonus points if your solution allows characters to get into conflict with each other about stuff like when a treaty ends or whether their loans are due 15 days sooner.