One year trailing median calculation

Trying to compute the trailing 365 day median for the following query. the "OneYearMedian" member isn't working. Any help would be very much appreciated:

WITH MEMBER Measures.OneYearMedian 
AS 
Median    ( {([FirstOrderDate].[Full Date].CurrentMember.lag(365):[FirstOrderDate].[Full Date].CurrentMember), [User].[User ID].MEMBERS}
      , [Measures].[Conversion Days]
   )


SELECT {[Measures].[Conversion Days], [Measures].[OneYearMedian]} ON 0,


NonEmpty(crossjoin([FirstOrderDate].[Full Date].members,
[User].[User ID].members),[Measures].[Conversion Days])
 
ON 1
FROM [UserRevenue]
WHERE ([FirstOrderDate].[Year].&[2011])