Not to belabor the point, but just to make sure I’m clear…
Isn’t each year a placeholder, e.g. 1990, 1991? And if P and M have to have one year between them then they can be next to each other and still have one “year” between them – there is 12 mos / 1 year between 1990 and 1991. Whereas if S and O have to have an element between them then they cannot be next to each other. Is that right?
You are welcome for my comments. They were certainly sincere.