VB Code to Calculate Query Results

I have a client billing database. If I'm on a Client Project form, I want to click on the New Invoice button and have the total amount of time entered for that project (within a given period) appear in a message box.

My guess is that a query will have to run that filters the Time table based on the Project ID and Start and End dates. How do you sum all of the hours in the resulting query?

The following details my thought process:
>Click on New Invoice, a form/message box appears that displays the current Project ID and prompts for for Start and End Dates.
>A query runs that filters the Time table based on the Project ID, Start and End dates.
>The sum of the resulting records is stored in a variable. [NEED HELP WITH THIS STEP]
>The variable is passed to a message box that says "do you want to bill Client for X hours?"
>The X variable is passed to the Subtotal field on the Invoice form.

I am an intermediate VBA user but I have no idea how to calculate the results of a query.

Any assistance is greatly appreciated! Thank you!

How are you planning to get hours from a start and end date? Are you billing a flat X hours per day?