Last Month calculation of selected Dates

I have a time dimension and a Last Month calculation.

In Cube Browsing, when I only select some dates in a month (e.g. 4th and 5th May 2007), at month level (May 2007), I found that the "Current Time" calculation gives me the sum of 4th May and 5th May. However, my LM calcuation gives me the total sum of LM value for the whole May 2007.

My LM calculation is as follows:

[Time].[Month Comparison].DefaultMember,
ParallelPeriod(
[Time].[Date with Month].[Month],
1,
[Time].[Date with Month].CurrentMember
)

Can anyone tell me how can I archive the calculation of LM at month level that only sum up the LM-value of the selected dates?

Thanks in advance.

Sorry, but I'm a little confused about what it is you are calculating. Could you provide more info, maybe a full MDX statement or query?

Thanks, Bryan