You have the option to author a contract with specific "service level agreements". For example, the well defined scope of work being completed by a specific date. The question then is what are the penalties or incentives for achieving or failing to achieve those SLA's.
My sense is it may be difficult to find a qualified contractor to agree to a punitive clause in the contract. Nor are they likely going to want to spend money on a lawyer to assist with reviewing and drafting a clause which releases them from liability for events that delay completion that are outside of their control.
Also, you probably don't want to run the risk of shortcuts and sloppy work resulting from a rush to get done or don't get paid mentality
So thats the contractual route.
My advice is to do your homework and choose a contractor with solid reviews and who's references consistently report accurate timeline and budget performance.
Source: http://HMDhome.com