I need to create a timesheet for myself to add up the hours I work

How do I set it up so the time is calculated itself rather than my having to
work it out.

I will have the following headings:-

Date
Time Started (ie 9.00am)
Break (ie lunch if taken)
Time Finished (ie 4.00pm)

In essence 09.00am - 16.00pm = 7 hours (less 1 hour for lunch)

Thanks