I figured out my own answer. Note that this is for item donations. The answer by troupee I think is for money donations as item donations don't give an option for single or multiple donations.
On the Home screen click the year's donation tab you want to work on, for example "2018 Donations." If you have more items to add for a different date click "Edit" in the items box. That brings up your "2018 Item Donations." Don't hit the "Add" button in the list of donations (or donation since I/we couldn't add another date) where "Add, Edit, and Delete" are, but click "Add Item Donation" under the donation totals and you'll then be able to start a new date.
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