You can enter a date prior to the current date in Zoho Books when you create an invoice. Do note, that the accounting period in which the invoice is dated must be before the lockout date (typically you close a financial year when the accounts have been reported). It is also best practice to not update transactions in a period where the GST has not been reported to the ATO yet via the BAS.

You should also be aware, that when you have uploaded the opening balances, this invoice may be part of that upload. To prevent double accounting for this invoice, ensure that either the original invoice was not part of the upload (this includes the ledger transactions pertaining GST and accounts receivable- not just the actual invoice), or make a reversal booking in the manual journal of the original invoice posting and associated GST postings in accounts receivable and GST ledgers.

To backdate an invoice, simply create the appropriate invoice in Zoho Books, and enter the date of the invoice when it was sent out with the previous package. Save the invoice as DRAFT. You then select in the menu MORE on the right hand side of the invoice the option MARK AS SENT. This places the invoice amount and GST in the appropriate Accounts Receivable and GST ledgers. You can then match the payment received against the invoice.