Hi Jason,
I don't know whether or not you've figured this out, but i think your MDX can and should do without the 'Existing'-piece. If your goal is to calculate the dates within the selected timeframe,just leave the Existing-pice out. Consider this, based on the AW cube:
with member [measures].[date count] as
descendants([Order Date].[Calendar].currentmember,[Order Date].[Calendar].[Date]).count
select {[measures].[date count]} on 0
from [Adventure Works DW]
where [Order Date].[Calendar].[Month].&[2003]&[1]
Works fine also when added to the cube.
Kind regards,
Cees