Moving Aggregator

Hello,

I want to use Moving aggregator: Backward; Window L 3; Mean. But I don’t want the last row to be taken into account (by date).

Example : 4 ; 8 ; 12 ; 20

I get (4 + 8 + 12) / 3 but i want (8 + 12 + 20) /3

If you have any idea

Thanks