EXCEL迷

C2=LEFT(A2,4)-(RIGHT(A2,2)-4<0)&LOOKUP(RIGHT(A2,2)*1,{1,4,7,10},{12,"03","06","09"})

另解=LOOKUP(A2-1,(LEFT(A2,4)-{1,0,0,0}&{12,"03","06","09"})*1)

另解=TEXT(EOMONTH(LEFT(A2,4)&"-"&LOOKUP(--RIGHT(A2,2),{1,4,7,10}),-1),"YYYmm")