I have the details you need about setting up the RTI date in QuickBooks Online (QBO), @john-iles.
We always make sure we're compliant with the HMRC guidelines. Thus, QBO automatically uses the date of the payroll run as the RTI date when sending the data to them. For more details, check out this article: Enabling Real Time Information (RTI).
When you create a pay run, you'll have to submit an FPS to the HMRC on or before the employee's payday. To learn more about this process, I'd suggest clicking a link below, depending on the payroll version you're using. Also, each article has more information on how to view your current and previous FPS filings.