Change the first part of your range in your calculation to Axis(1).Item(0)

So in AdventureWorks the example would be:

WITH

MEMBER Measures.toto AS

Aggregate({Axis(1).Item(0):[Date].[Date].currentmember},

[Measures].[Order Count])

SELECT

{[Measures].[Order Count], Measures.toto} ON 0,

NON EMPTY {[Date].[Date].&[20060301]:[Date].[Date].&[20060331]} ON 1

FROM

[Adventure Works]