Imagine explaining how we measure time to aliens. "Yeah, months are thirty days. Some are thirty-one. One's twenty-eight, but every four years, it's twenty-nine."
When I was younger I didn’t understand how time worked or that months had an order, so every few weeks I would ask if it was my birthday month yet because maybe it would just pop up because while I ostensibly knew how to count, in reality it seemed like dates were arbitrarily determined by the Society of Adults Dedicated to Thwarting Kayt’s Desire For Many Presents and to the aliens you describe that’s kinda what the entire calendar is like.