Calcukate no of months between dates

Hey guys,

I am stuck with a simple problem.

I need to get the total number of months between 2 dates.

I have attached my sample file.

I need to substract Range "E" and "N".


So far, I got

* number of years in Range "R"
* number of months in range "S"
* number of days in range "T"

I have added range "R" and "S" to get total number of months in range "AC"


However i am stuck with the days .



If the number of days (Range "T") is above 2 and equal to less than 17 then it must be considered half month
If the number of days (Range "T") is equal to greater than 18 than it must be considered 1 month


Thus,
for example
if number of days is 2, then its 0 months
if its 6 days then i consider it half month (0.5 month)
if its 19 days then i consider it 1 month



Anyone can help me please?

Its basically for calculating interest on morgages