Use the DateTimePicker control to select the date on which this transaction took place. For instance, if today is Tuesday, October 17, 2017, and you just received an email from PayPal showing a payment made by the customer whose name appears on this form, you'd choose 10/17/2017 as the transaction date.

The transaction date is the date on which the PayPal payment gets deposited, or the date on which you cash your customer's personal check, or the day on which they hand you an envelope full of cash. Etc.