When recording old invoices, let's make sure that we enter the correct dates. From the + New button, click on the Invoice. This link will guide you in recording invoice payments in QuickBooks Online. You can also import them using an Excel or CSV file. One file can have a maximum of 100 invoices and 1,000 rows. Please refer to this article for more details: Import multiple invoices at once.
If this is your first time using QuickBooks Online, then recording the old transactions is fine and won't affect your data. Just make sure to start with the oldest ones. I've added this article about the reconcile workflow in QuickBooks.
You can reply anytime if you have other clarifications. I'll always be here to help you.