>How to get the number of months when date1 is substracted from date2?

First of all it depends on what you mean by month.

1) 30 days

nMonths = (dDate2 - dDate1 )/30

2) 28 days

nMonths = (dDate2 - dDate1)/28

3)Or you could calculate something like this. If someone were to ask how many calendar months have you been in business. Example: if you started your business on Jan 15 and it was now Feb 8. You might respond, I have been in business 2 months (Jan and Feb)

nMonths = (year(dDate2)*12 + month(ddate2)) - (year(dDate1)*12 + month(dDate1)) + 1

What is it that you are trying to calculate?