Moving Average of Last 24 months

Question

I have this calculated member which calculates a moving average for the last 12 months:

iif(IsEmpty(Sum({[Time].[Month].CurrentMember:NULL},

[Measures].[Count])), NULL,
Avg
(
   [Time].[Month].CurrentMember.Lag(11) : 
   [Time].[Month].CurrentMember, 
   [Measures].[Count]
))

The iif condition is in place because I don't want to get values for future months (with no value), which I do get without it.

What I want to do is have this measure only for the last 24 months since the last not empty month.

I've tried with Tail and Lag but with no luck (I would post my attempts here but after many tries I deleted them and would really not know where to begin again).

Thanks to @whytheq this is the final solution that I used:

CREATE DYNAMIC SET CURRENTCUBE.[FirstEmptyMonth]
 AS { Tail
      (
        NonEmpty
        (
          [Time].[Month].MEMBERS
         ,[Measures].[Count]
        )
       ,1
      ).Item(0).NextMember };        
CREATE DYNAMIC SET CURRENTCUBE.[MonthsToIgnore]
 AS {[FirstEmptyMonth].Item(0) : NULL}
    + 
      {NULL : [FirstEmptyMonth].Item(0).Lag(25)} ;                
CREATE MEMBER CURRENTCUBE.[Measures].[Moving Average]
 AS IIF
    (
      Intersect({[Time].[Month].CurrentMember},[MonthsToIgnore]).Count = 1
     ,null
     ,Avg
  (
    [Time].[Month].CurrentMember.Lag(11) : [Time].[Month].CurrentMember
   ,[Measures].[Count]
  )
    );

Answer 1

In AdvWrks I've got this:

WITH 
  SET [FutureMonthsWithNoData] AS 
    {
        Tail
        (
          NonEmpty
          (
            [Date].[Calendar].[Month].MEMBERS
           ,[Measures].[Internet Sales Amount]
          )
         ,1
        ).Item(0).NextMember
      : NULL
    } 
  MEMBER [Measures].[blah] AS 
    IIF
    (
        Intersect
        (
          {[Date].[Calendar].CurrentMember}
         ,[FutureMonthsWithNoData]
        ).Count
      = 1
     ,null
     ,1
    ) 
SELECT 
  {
    [Measures].[Internet Sales Amount]
   ,[Measures].[blah]
  } ON 0
 ,[Date].[Calendar].[Month].MEMBERS ON 1
FROM [Adventure Works];

It returns this:

So what I am saying is that you could create this initial set of FutureDatesWithNoData and then use that set to create a condition within your script. The set would be (I think) this in your cube:

SET [FutureMonthsWithNoData] AS 
{
    Tail
    (
      NonEmpty
      (
        [Time].[Month].[Month].MEMBERS
       ,[Measures].[Count]
      )
     ,1
    ).Item(0).NextMember
  : NULL
} 

Your measure would then be as follows:

IIF
(
    Intersect
    (
      {[Time].[Month].CurrentMember}
     ,[FutureMonthsWithNoData]
    ).Count
  = 1
 ,NULL
 ,Avg
  (
    [Time].[Month].CurrentMember.Lag(11) : [Time].[Month].CurrentMember
   ,[Measures].[Count]
  )
)

If you want to also exclude months prior to 24 months ago then this script sums up the logic:

WITH 
  SET [FistEmptyMonth] AS 
    {
      Tail
      (
        NonEmpty
        (
          [Date].[Calendar].[Month].MEMBERS
         ,[Measures].[Internet Sales Amount]
        )
       ,1
      ).Item(0).NextMember
    } 
  SET [MonthsToIgnore] AS 
      {[FistEmptyMonth].Item(0) : NULL}
    + 
      {NULL : [FistEmptyMonth].Item(0).Lag(24)} 
  MEMBER [Measures].[blah] AS 
    IIF
    (
      Intersect({[Date].[Calendar].CurrentMember},[MonthsToIgnore]).Count = 1
     ,null
     ,1
    ) 
SELECT 
  {[Measures].[Internet Sales Amount]} ON 0
 ,[Date].[Calendar].[Month].MEMBERS ON 1
FROM [Adventure Works];