counting months in irregular fiscal year

counting months in irregular fiscal year

If anyone out there knows of an elegant way to count the number of months since the beginning of an irregular fiscal year I would really appreciate send that gem my way.

I had an app all ready to go for a client that assumed a regular year of Jan1--Dec31 and used the basic .getMonth( ) date function to generate a number count since the beginning of the year for a given date. Then, the client informs me that oops...their fiscal year is May1-Apr31. Now my app formulas must be changed to reflect this structure:
 
May, month 5 is the number 1 month since year inception,
June, month 6 is the number 2 month since year inception,
July, month 7 is the number 3 month since year inception,
etc.

Please post any helpful hints.

Thanks