Either format the cells to just DD format, OR, use this

=IF(MONTH(DATEVALUE(F4&" "&D4))<>MONTH(DATEVALUE(F4&" "&D4)-(WEEKDAY(DATEVALUE(F4&" "&D4))-1)+{0;1;2;3;4;5}*7+{1,2,3,4,5,6,7}-1),"",DAY(DATEVALUE(F4&" "&D4)-(WEEKDAY(DATEVALUE(F4&" "&D4))-1)+{0;1;2;3;4;5}*7+{1,2,3,4,5,6,7}-1))