You can update the number of days that payments are backdatable by by following the below steps:
Go to 'Admin' > 'Practice' > 'Practice Details' > Update Payments Backdatable By Field. See below:
This figure can be updated up to 999 days.
After you have updated this field you will then be able to backdate payments by adding them through as normal and amending the payment date. For example, if you have set the 'payments backdatable by' field to 30 you will be able to backdate the payment date up until 30 days. See below: