I would look into cross-tab queries, that is a good way to monitor a total on running values.
Once you get a query that will search for the right record, and give you the total hours, simply create your form to include your table and your query. Use the controls you need to edit data on your table, and then us the query result as your running total on your form. If done this way you may not even need a cross-tab query to do it, simply a query that searches for the record, and add a field which is the sum of the hours worked.
This all assumes that you DO have it working where when you enter the two times the time on that job DOES calculate correctly. For example, when you enter 8:00 and 10:00 the table automatically enters 2 or 2.0 for the hours worked on that specific job. If that works, running a sum of those hours worked should be easy. It might even be easier to make that field a number field rather than a date/time field, as it only shows the difference, and you want your results in a format with a decimel (8.4 total hours worked) rather than time/date format (8:40 worked).
Hope this helps!
If this doesnt work for you, attach your DB or take a screenshot of your table, with data in it, and I will look at it and try to get an idea of what you need.
