Issue with contractor: works too slow!

[Michael Ream]

So I hired a contractor to refinish a basement: frame, drywall, skim coat and paint walls, paint ceilings (joists) and concrete floors, add a bathroom and laundry room, update electric. Work is going well enough, but rather slow: signed contract includes a section that states "substantial completion" will be done on or before 35 working days after work begins. Today is the 37th working day, and it doesn't look like they'll be done anytime soon.

My question: for future projects, is there language I can put in the contract of penalty(ies) (monetary, etc.) if the contractor has not completed the project by the date listed in the contract? Interested in hearing others' experiences and how they enforced such penalties.