Dynamic Dates as per User Response

Hi Experts,

In my report , i want to display the dynamic dates as per user response,

No Universe..

Eg-1, In webi Prompt, User select any date between (20141125 - 20141225). Iam selected as - 20141201, we want to display in report header like (11+1) where 11 is month names = jan, feb, mar, apr, may, jun, jul, aug, sep, oct, nov and 1 is dec as forecast -> 11+1

Eg-2 User select any date between (20141225 - 20150125). Iam selected as - 20150101, we want to display in report header like (12+1) where 12 is month names = jan, feb, mar, apr, may, jun, jul, aug, sep, oct, nov,dec and 1 is jan as forecast -> 12+1

I tried using case statment with between operator based out of user response date, but struck with formula..not working as expected..

Appreciate with the formula help..

Thanks,

-Dinya.