After updating to WHMCS 8.12.1, admins may encounter an Oops! error when placing an order for a product with the Prorata Billing feature enabled and a Prorata Date setting of 0:

ERROR: Carbon\Exceptions\OutOfRangeException: day must be between 0 and 99, -1

To work around this issue:

1. Go to Configuration > System Settings > Products/Services.
2. Click Edit for the desired product.
3. Select the Pricing tab.
4. Uncheck Prorata Billing.
5. Click Save Changes.

A product with a Prorata Date setting of 0 will work in the same way as an anniversary-billed (non-prorata) product. Because of this, disabling Prorata Billing will not change how WHMCS handles the product.