The business requirements document should accurately, and in detail, describe the purpose of the project. It states what is needed to achieve that goal i.e. what is in-scope, what is out-of-scope, any assumptions that have been made, any constraints that have been imposed and expected timescales.
The timescales must be realistic and, in practise, the project manager will probably have discussed timescales with the team before the schedule was finalised. They will also need to understand the criteria by which each task is determined as complete.