There may be an easier way but I would do this
Create two date parameters, start and end
In selection criteria add
datefield between {?Start} to {?End} and
datefield between dateadd("yyyy", -1,{?Start}) to dateadd("yyyy", -1,{?End})
Create two formuals which return your amount values for the two date ranges, place in details and then sum each one, suppress details, you will then have two totals for each date range which you can display or use in a formula to calculate %age change.
The dataadd formula will not be passed to SQL statement so you will bring back all data to report so if you have a big database it will be slow to run.
Ian