One way this can be done is to place the Sale Date and Customer fields on the rows shelf. Then, create a calculated field similar to the following:
Place this calculation on the view, right click and select Compute Using-->Sale Date. Then, create a second calculated field:
if attr([Sale Date])<=#6/1/2010# and [Calculation1]=1 then sum(Sales)
elseif attr([Sale Date])<=#12/30/2010# and [Calculation1]=1 then sum(Sales) end
Place this on the view. Remove Customer from the rows shelf and place it on the level of detail shelf. Remove the first calculation from the view.
Hope this helps a little bit!