How to use YTD on a mirror period

Hi friends

I have a funny situation. I have a period dimension which contains Year, Qtr, Month. I also have another Time dimension which contains Mth1 .. Mth25.

The 2nd Time dimension matches that of the 1st Time dimension ie. Mth25 is 2005.April. and Mth24 is 2005.March and so on...

However I need to calculate YTD when MTH25 is selected from the dimension.

I can't exactly do YTD (Mth25) as Mth25 is not a time dimension. Is there a way to Retrieve the equivalent time dimension in the form of 2005.April from Mth25 so that I can use in YTD Calculation ?

Thanks
Tom