Agreed. I, by no means work construction, but have some buddies that do. I believe that when the company seeks out a construction company they meet with a project manager and tell the project manager the time line in the meeting, with a goal date typically ahead of the "go live" date to allow for any unexpected events (weather, etc...) I don't know if a company would really be able to succeed if they just said "Eh, let's finish it whenever we get the chance, if not this year, hey maybe next year we will get it."
It will usually be pretty specific as far as project timeline. There will be an Owner/Contractor Agreement, which stipulates a Notice of Commencement date...starting at this date the contract will stipulate an amount of time, (usually in days) upon which a Contractor will achieve substantial completion. Sometimes but not always, written into the contract there may be penalties for days after the set completion date that a contractor does not complete the work, that the contractor will have to pay a penalty for each day beyond the set date. This is usually due to lost revenue due to the project not being completed, such as in the case of rental units (or possibly a ride which will draw guests and increase revenue) On the other side of that, there is also sometimes a bonus for work completed ahead of schedule. The schedule will typically shift due to weather or owner directed changes, but typically there is always a set date of completion.
Hope that helps.