Quite a few invoices that have been paid by the client but not paid off against the invoices, thus my AR i is wrong, can I pay them off backdated, some go back 12months?

I can see that you want to backdate a payment for an invoice, @mark345. I'll be happy to share information on how to accomplish it in QuickBooks Online (QBO).

Yes, you can. If the books are closed and you want to receive a payment for an invoice from that period, we'll need to edit the closing date first so you can make changes. To do that:

1. Go to the Gear icon and select Account and Settings.
2. Click the Advanced tab.
3. In the Accounting section, click Edit.
4. Modify the closing date. Select Save, then Done.
   

After you change the closing date, you should be able to mark invoices as paid. However, please keep in mind that this can affect your financial statements, so it's best to consult with your accountant before making any modifications. 

Additionally, if you've received bank deposits from your customer, you may refer to this article on how to link them to an invoice to mark it as paid: Link a bank deposit to an invoice in QuickBooks Online. 

Furthermore, QBO offers dozens of reports to help you obtain an overview of your business. This way, you'll be able to see how your sales and expenses are performing.

Please know that the Community is open around the clock whenever you have any follow-up concerns about backdating a payment for an invoice. Just mention me in your reply, and I'll sprint back into action. Stay safe and have a wonderful day ahead!