I would also add this tid bit. You can also include penalties for not completing on time. For example; You can say,,,If thus and such is not completed a reduction in cost of $250.00 per day will begin after the first 24 hours and will include those 24 hours as part of the penalty.
I did this with a roofing contractor who was giving me a huge big deal. The new roof was to cost $5,000.00 finished. This guy would take off days, work slowly, smoke, mess around, or just not show up. By the time he was finished I owed him a total of $300.00 which was less than the cost materials.