How do I add a late fee at midnight on a certain date?

You may use a time field and conditional logic to apply the fee based on the time. 

First, add the time field from the add form element section on the form builder, then click on the field settings and set the time to the current one. 

You will need to add a new additional field, it may be a short text entry field, where you will insert the fee.

The next step is to include this field on your calculations for the total due, and finally you can set up the condition to insert the value based on the time field, here is a guide that will help you setting up this type of conditions: https://www.jotform.com/help/268-How-to-Insert-Text-or-Calculation-into-a-Field-Using-Conditional-Logic

If you have questions, let us know. 

I must be missing a step because it is not adding the $10 - where do I put
the $10 - I am using paypal and adding products. There is no total field on
the form to pick from...so I am confused it is this form if that matters:
https://form.jotform.com/pamball/pride-basketball-registration-2020
...

Unfortunately, this will not work while using the sell products payment type, instead you will need to change it to "user defined amount", you can change this on the gateway settings, to do so, click on the payment field while editing your form and click on the wand icon. 

Now, you will need to set up your product using a basic field such as a checkbox, add a calculation value and manually perform the calculations, then you can pass the calculation to the payment field. This way you can add an additional short text entry field where you will conditionally insert the fee as explained above. 

The following guides will help you doing so: 

https://www.jotform.com/help/301-How-to-Assign-Calculation-Value

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget

https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field

I hope this helps. 

ok thanks! I think I may just change it manually this time. We had some
issues with PayPal having it set up the other way so I will just add the
$10 manually at the cut off time but I will keep these directions for next
time to give it a try! Thank-you!
...

Sure. Feel free to get back to us if you have any questions and we will also help you build a demo form on how to achieve your requirement.