Increasing or Adding Month in Salesforce Formula Field

Problem :

Here is the scenario of my requirement,

Custom Fields:

API Name: Purchased_Date__c

Data Type: Date

API Name:  Expire_Date__c

Data Type: Formula

Return Typ: Date

For example,While creating record if the user gives date as 08/03/2013 in Purchased_Date__c field and the Corresponded formula field should return after 3 months date from the Purchased Date.

If i give Purchased Date as 09/13/2013(MM/DD/YYYY) means the Expire Date will be 12/13/2013(MM/DD/YYYY)

Solution: 

1. DATE(
2. year(Purchased_Date__c)
3. + floor((month(Purchased_Date__c) + 3)/12) + if(and(month(Purchased_Date__c)=12,3>=12),-1,0)
4. ,
5. if( mod( month(Purchased_Date__c) + 3, 12 ) = 0, 12 , mod( month(Purchased_Date__c) + 3, 12 ))
6. ,
7. min(
8. day(Purchased_Date__c),
9. case(
10. max( mod( month(Purchased_Date__c) + 3, 12 ) , 1),
11. 9,30,
12. 4,30,
13. 6,30,
14. 11,30,
15. 2,if(mod((year(Purchased_Date__c)
16. + floor((month(Purchased_Date__c) + 3)/12) + if(and(month(Purchased_Date__c)=12,3>=12),-1,0)),4)=0,29,28),
17. 31
18. )
19. )
20. )