一応できたけど、式が汚すぎる
=TEXT(DATE(YEAR(DATE(YEAR(T2),MONTH(T2)+(DAY(T2)>20)-(U2<>""),1))-(MONTH(DATE(YEAR(T2),MONTH(T2)+(DAY(T2)>20)-(U2<>""),1))<=2),
MONTH(DATE(YEAR(T2),MONTH(T2)+(DAY(T2)>20)-(U2<>""),1)),1),"yyyy""_""mm")