On the following MDX, the Bill date 2009-10-28 does not exist so
[Measures].[91-120] returns null for all membersin the column. But
2009-10-29 does exist. But let's say I did not know that. How can I
write this so that the query finds and uses 2009-10-29? I tried
playing with HEAD and DESCENDANTS and some other functions but I can't
seem to figure out a way to do this. Is it possible?

With member [Measures].[91-120] as
sum([Dim Bill].[Bill Date].&[2009-10-28T00:00:00] : [Dim Bill].[Bill
Date].&[2009-11-26T00:00:00],
[Measures].[Balance])

select {[Balance],[Measures].[91-120]} on columns,
Filter(([Client Billing Employee].[Full Name by Last Name].members),
[Measures].[Balance]<>0)
on rows
from [AR]
where (Null:[Dim Time].[Date].&[2010-02-28T00:00:00])

Thanks in advance for any assistance.

MDX decendants function is nicely with more examples and diagrams on the below location.
http://www.sqllion.com/2011/08/mdx-decendants/