>>486
月末日でカウント＋退職日空白もカウント

=LET(
x,EOMONTH("2025/10/1",SEQUENCE(20,,0)),
y,COUNTIFS(B$1:B$100,"<="&x,C$1:C$100,">="&x)+COUNTIFS(B$1:B$100,"<="&x,C$1:C$100,""),
HSTACK(TEXT(x,"yyyy/mm/dd"),y)
)