In order to see giving totals for a custom time period, you'll need to run a Gift query to identify the right Gifts, export your file to a csv, and then use the Subtotal function in Excel.
Then, save your query with a name (i.e., "Last Fiscal year Gifts") and then click on Select Export fields. You may select any fields that you like, but be sure to include the Contact ID and Gift Amount fields. Click "Save," and then click on the "Export CSV" button to begin your export.
Once you file is ready, you can download it and open it in Excel. There, it is recommended that you sort your file by Contact ID, to make sure all Gifts from each giver are grouped together. Then, you'll want to use the Subtotal option on the Data ribbon to generate your totals.
The Subtotal dialog box will provide multiple options. Be sure to subtotal at each change in Contact ID, and to use the Sum function on the Amount column.
Your file will then contain giving totals for each giver, identified by Contact ID.