Hi Harish, you can try to see the help document in BP, it enum the list of the interval for both DateAdd() and DateDiff(), the index is start wiht 0.
Interval DateAdd
0 Year
1 Week
2 (n/a)
3 (n/a)
4 Quarter Quarter
5 Month
For example, to add a number of months to a date use interval number 5. So to add 2 months to 26/5/1999, use DateAdd(5, 2, MakeDate(26,5,1999)). This would correctly return 26/7/1999.