Calculating time, potential lunch break

Hi All

I'm struggling with a formula.

I am creating a worksheet where employees schedule work throughout the day
eg task 1 will be start time 9am, finish time 11am. The result is 2 hours to do the task.

The problem i'm having is that I need to do a calculation that if a task includes lunch then the result takes an hour away from the task time.
eg task 2 will be start time 12pm, finish time 3pm, excel displays 3 hours, i need it to say 2 hours because 1-2pm is contractual lunch.

I can't just do a day formula that says minus 1 from total as this is task orientated and will vary depending on start and finish times of each task.

I hope someone can help.
PS, it needs to be a formula and not vba as we have a problem with macro's if multiple people use the worksheet.