MDX Filter set by Date inside a set

Hello again,

I'm having trouble to filter a set with a condition that verifies if the calculated member(Date) is part of an date set.

I really don't know how to write this query, so I'll post what I wish worked(using the IN command).

SELECT {
	[Measures].[Valor Item], 
	[Measures].[First Activity]
} ON 0,
{
	Filter(
		[CLIENTE].[Cliente].[Cliente].ALLMEMBERS,
		[Measures].[First Activity] IN {[Time].[Time].[Year].&[2011-01-01T00:00:00]} -- That's the trick I'm trying to do
) ON 1

First Activity is the first date that the customer has bought something. 

The point of this query is to show only the customers that the first buy was in the selected time range.

How could I make this happen?

Thanks.