I am not sure exactly what you are trying to accomplish.
So this is your table?
Year Month EndingMonth Credit Debit
2015 1 1/31/2015 100 300
2015 2 2/28/2015 100 300
2015 3 3/31/2015 100 300
2015 4 4/30/2015 100 300
2015 5 5/31/2015 100 300
2015 6 6/30/2015 100 300
2015 7 7/31/2015 100 300
2015 8 8/30/2015 100 300
2015 9 09/31/2015 100 300
2015 10 10/30/2015 100 300
2015 11 11/31/2015 100 300
2015 12 12/30/2015 100 300
Then you want to run the report with selection january, and a month later feb...
If you want to calculate the YTD in febraury, you also need the data of january in your selection, so you might want to use suppress.
Do you know how to suppress?
In the section expert, details, click on the formula button from 'suppress (No Drill-down)'.
Enter the formula
Courier[Month} <> {?Month}
Where {?Month} is the parameter field in which you choose the month.
Is this helpfull?