If performing this in the UI, you go under Subscriptions > Invoices > Create an Invoice. There is a new input field for Issue Date in the top right of the ad-hoc invoice within the UI. The issue date will be set by default to the current date. In order to back date the issue date, you will use the date selector functionality by clicking into the Issue Date input field.
Alternatively, you can also backdate the issue date via the API with the "Create an Invoice Endpoint". If no issue date parameter is passed, then the issue date will default to the current date. If you desire to back date, you can simply pass an issue_date parameter of any date in the past.
The primary use case for backdating ad-hoc invoices is when you experience a delay in issuing an ad-hoc invoice but would still like for the invoice to be due at the same time it would have been had there been no delay. As an example, one-off usage is incurred but not charged until 7 days later and you want the due_date to reflect your standard net terms of 15 days. In this case, you could back date the ad-hoc invoice to the day the usage was incurred and set the due_date to Day 15.
For more information on backdating the issue date on ad-hoc invoices, see our documentation here>