If you are using date parameters for the start and end dates, you could do something like the following:
First, for your record select statement, use something like:
{table.date} in Date(year({?start})-1, month({?start}), day({?start})) to {?end}
Then for this year's performance period, use:
if {table.date} in {?start} to {?end} then {table.sales}
For last year's performance period, use:
if {table.date} in Date(year({?start})-1, month({?start}), day({?start})) to Date(year({?end})-1, month({?end}), day({?end})) then {table.sales}
You can insert summaries on these to obtain results at any group level you may have.
-LB