MDX - Getting the last 6 month Dynamically
Assuming you have properly set the ValueColumn property of your CreatedOn attribute in your dimension (so that MemberValue comes back a date data type) then the following should work:

select {[Measures].[Your Measure Here]} on 0,
[DimAccounts].[AccountPK].[AccountPK].Members on 1
from [Your Cube]
where {
 Filter(
  [DimAccounts].[CreatedOn].[CreatedOn].Members
  ,[DimAccounts].[CreatedOn].CurrentMember.MemberValue &gt; DateAdd("m",-6,Now())
 )
}


Or if you want to create a named set so that any user can just drop it on rows, put this in the calculations tab of your cube designer:

CREATE SET CurrentCube.[Accounts Created In Last 6 Months]
as
Exists(
 [DimAccounts].[AccountPK].[AccountPK].Members,
 Filter(
  [DimAccounts].[CreatedOn].[CreatedOn].Members
  ,[DimAccounts].[CreatedOn].CurrentMember.MemberValue &gt; DateAdd("m",-6,Now())
 )
);