java script code to calculate timesheet

I have a timesheet form that adds and populates based on a varied work week.  Normal work week hours can vary by each employee.  I want to auto-populate two boxes "Regular Hours" and "Overtime Hours" based on whatever is entered in the "Time in and Out" field.

Regular work hours is 8 after that count as Overtime.

Ex:

I would greatly appreciate any assistance with this.

Thanks!