For the past couple months if I change the due date of an invoice from anything but the default 30 days it gives me an error and I cannot email the invoice.

I understand that you have been having trouble changing the due date of your invoices in QuickBooks for a couple of months, @kseitzdvm. I'm here to provide you with troubleshooting steps to resolve the issue. 

First, let's use a private browser or an incognito window upon logging in to your account to isolate the hurdle. This mode is free from cache files that avoid storing internet data.

Here’s how:

• For Chrome, press and hold Ctrl+Shift+N.
• For Firefox, press and hold Ctrl+Shift+P.
• For Safari, press and hold Command+Shift+N.

Once you've successfully logged in to your account, clear the cache of the browser you're currently using. You can refer to the Clearing cache and cookies to fix issues article for detailed guidance.

If you're still having trouble, you can use a different browser since every browser uses a different cache storage folder on your computer.

In case you want to process your invoices in QBSE, you can visit this article(Section 3) for guidance: Create invoices in QuickBooks Self-Employed.

I'm always here to assist you with any other concerns you may have regarding managing and modifying your invoice dates. Please don't hesitate to reach out again,