付款日為每5日 (13986)

C2=IF(DAY(B2)>24,EOMONTH(B2,0)+5,TEXT(B2,"YYY/M")+CEILING(DAY(B2)+1,5)-1)

另解{=IF(DAY(B2)>24,EOMONTH(B2,0)+5,B2+MATCH(,MOD(DAY(B2+ROW($1:$5)),5),))

另解=EOMONTH(B2,-(DAY(B2)<25))+FLOOR((DAY(B2)<25)*DAY(B2)+5,5)