Hey evgeny.gozenpud,
Recording your transactions with the correct dates is crucial for unlocking the full potential of QuickBooks Online. This ensures consistent reporting, and helps you file your sales tax returns accurately. As the credit memo will affect your profit and loss in the period it's recorded, it's important to select the appropriate date. In general, transactions should be recorded with the date they actually occurred on. If you're unsure which date this is, your accountant is the best person to ask. They'll be able to make sure everything is correct for tax purposes. You can easily invite your accountant as a user to approach this together. To search for a pro in your area, simply head to the My Accountant tab in QuickBooks Online then click on Find a pro to help.
Hope this helps!