You can edit the date of your rounds after saving them. That's a nice feature for when you forget to record a round or if you simply want to add past rounds to the Hole19 app.
Go to the 'Performance' tab and tap 'View All' to see all previous rounds. On the Round Summary tap the 3-dots icon on the top-right and then 'Edit Date'. Select the correct date and tap 'OK' to save the information.