Assuming your "date begin" and "date end" columns are valid JMP Date/Time values, then the new formula for calculating the difference is
Date Difference( :date begin, :date end, "month" )
Here is an image of the attached example data table that has the date begin and date end columns and a new column, Difference that uses the above formula
If your date begin and date end are Character columns and not JMP Date/Time columns, you will need to convert them to Date/Time columns before being able to calculate the difference. This can be done easily and has been covered multiple times in previous Discussion entries.
Jim