How do I create a Rolling 12 Months Accumulated Sum (InternetSalesAmtR12Acc) that can show a trend without seasonal variations?

WITH MEMBER [Measures].[InternetSalesAmtYTD] AS SUM(YTD([Date].[Calendar].CurrentMember),[Measures].[Internet Sales Amount]), Format_String = "### ### ###"
MEMBER [Measures].[InternetSalesAmtPPYTD] AS SUM(YTD(ParallelPeriod([Date].[Calendar].[Calendar Year],1,[Date].[Calendar].CurrentMember)),
[Measures].[Internet Sales Amount]), Format_String = "### ### ###"
MEMBER [Measures].[InternetSalesAmtPY] AS SUM(Ancestor(ParallelPeriod([Date].[Calendar].[Calendar Year],1,[Date].[Calendar].CurrentMember),[Date].[Calendar].[Calendar Year]),
[Measures].[Internet Sales Amount]),Format_String = "### ### ###"
MEMBER [Measures].[InternetSalesAmtR12Acc] AS ([Measures].[InternetSalesAmtYTD]+[Measures].[InternetSalesAmtPY] )- [Measures].[InternetSalesAmtPPYTD]
Select {[Measures].[Internet Sales Amount], Measures.[InternetSalesAmtYTD], [Measures].[InternetSalesAmtPPYTD],[Measures].[InternetSalesAmtR12Acc]} On 0,
[Date].[Calendar].[Month].Members On 1
From [Adventure Works]
Where ([Date].[Calendar Year].&[2004])