Let me sort things out for you, @richard-rhodes-p.
I appreciate you performing some steps to fix this. Normally, you can change the custom dates in QuickBooks Online. Since you're still unable to do the task after deleting and reinstalling the app, I suggest contacting our support team. This way, they can further check your account and help fix this.
In the meantime, I recommend signing in to your QuickBooks Online account using a browser. By doing so, you can change the date on the custom reports.
I've also added these articles that can guide you perform some task in viewing and accessing your reports in QBO:
Let me know if you have further queries with managing reports in QuickBooks Online. I'd be glad to help you. Wishing you a good day.