I'd like to share some information, Janfromearth1.
At this time, the date warning option for the future transaction is only available in QuickBooks Desktop. In QuickBooks Online, you can lock past transactions by closing the books. This is to prevent any changes made to transactions dated earlier than the closing date.
To add a closing date and password, you can follow these steps:
- Go to the Gear > Account and settings.
- On the left panel, click Advanced.
- Select the Accounting section, and click the toggle icon to enable Close the books.
- Enter a closing date in the field.
- Choose Allow changes after viewing a warning or Allow changes after viewing a warning and entering password from the drop-down menu.
- Click Save and then Done.
I've got this sample screenshot for a visual guide:

Here's an article for more details: Close your books to lock past transactions.
We're always available if you have more questions about QuickBooks Online. Take care and stay safe.