Current Reporting Month and YTD calculations

Hi.

I'm trying to design a report in Excel based on a cube that will default to the current reporting month (which is actually last month), but still allow the user to select a different month to report on.  My date dimension table has a column called CurrentRepMonth, which is set to 'Current Reporting Month' for all days in the previous month.

My date dimension has a hierarchy of Year - Month - Date, and an attribute called 'Current Reporting Month'.

I have a calculation in my cube to get the YTD hours worked, based on the selected month:

CREATE MEMBER CURRENTCUBE.[Measures].[YTD - Hours Worked]
 AS AGGREGATE ( YTD ([Date].[Year -  Month -  Date].CurrentMember), [Measures].[Hours Worked]), 

In excel, if you select a month in the filter, the YTD calculation worked.  However, if you filter by Current Reporting Month, the calculation returns blank.

Any help is much appreciated.