In order to be able to add bookings for past dates in the POS, ensure that the Book non-configured availabilities and custom time option is active in your Dashboard, under POS Configuration → POS Options.
To add a new booking for a past date, click the +New booking button and select the desired past date, just as you would when creating a future booking.
You will see the two warning messages shown below. These do not prevent the booking from being created; simply proceed with the booking process and manually add the date, time, and number of tickets.