There's a reason that the Money Pit scenario is such a home construction cliché. It's all too common for contractors to push that completion date back over and over. There are definitely some scenarios where pushing back the completion date makes sense. If a snowstorm shuts down the whole city on the same day you're supposed to break ground or a severe thunderstorm damages some of the work that they just completed, it's really out of your contractor's hands. Acts of God aside, though, there aren't many good reasons that a home construction project should take months or even weeks longer than predicted.
Financial incentives for on-time completion should help. The contractors who built our home addition gave a firm completion date as part of our contract, and for each day that they were working beyond that finish date, they had to pay us a fixed amount. During the particularly stressful and dusty parts of construction, we could keep our eyes on the prize, because we knew they had to finish by that set date. There's nothing like a financial incentive to get a home renovation project moving efficiently!
Not all contractors will guarantee a completion date, but if you can find one who will, you can save yourself a lot of headaches. A solid contractor will give himself plenty of padding for that date to account for the unexpected, and knowing when the work will absolutely be done is great for your peace of mind.