Hello,
You can test following :
=SUMPRODUCT(--(DAY(A1:A12)>15))
HTH
=DATEDIF(A1, B1, "d") will give the difference in days between A1 and B1
To add a month to a date you can use =EDATE(A1,1) So stick first month in A1 and =EDATE(A1,1) in A2 and drag the formula down to A12. Then stick =IF(B1<>"",DATEDIF(A1, B1, "d"),"") in cell C1 and drag it down to C12, use Column B to enter actual date received