Thanks for posting in the Community, JSELL1750!

If you are using the QuickBooks Online Essentials and Plus version you can make use of the custom field option in QuickBooks Online to be able to add BILLED period in your sales form. Here's how:

1. Click on the Settings icon, and click on Custom fields.
2. Click on Add field or Add custom field.
3. Enter a name in the Name field.
4. Tick the All Sales forms or Purchase Order box.
5. To show the custom field on printed and delivered forms, turn on Print on form. Otherwise, it’ll only appear in QuickBooks Online.
6. Select Save.

Then, create a dummy invoice to see newly created custom field. 

Furthermore, if you are billing your clients on a quarterly basis you can also set up Quarterly payment terms so your due date will be set quarterly. Here's how to add new terms:

1. Click on the Settings icon.
2. Click on All List.
3. Click on Terms, and click on New.
4. Enter a name, and under Due in fixed number of days you can enter 90 or depends with your invoice terms.

When you create an invoice set the terms of how long the customer needs to pay the invoice.

For reference you can also check this link: Add custom fields to sales forms and purchase orders in QuickBooks Online.

Post again in the Community if you have further concerns. I am around to help you.