"One month" is a tricky unit because it's not always the same size. They have a good example right there. What's a month after Jan 31? Feb 28 or 29. A month aft

"One month" is a tricky unit because it's not always the same size. They have a good example right there. What's a month after Jan 31? Feb 28 or 29. A month after that? Mar 28 or 29. What's two months after Jan 31? Mar 31. Maybe you think it should behave otherwise, but they picked a way and changed the name so you wouldn't think of the addition operation and its expected properties.