A project can be scheduled, but the timing of acceptance + bug fixes is more difficult to plan. Not to mention the additional requirements, some of which the contractor has to do within the framework of the main body of work. How can you plan deadlines, especially when they are required by your/other management?
An extended combustion diagram will help, see figure.
How it works:
From left to right - iterations (or weeks, as convenient) of work.
All that is above zero - work on the TOR (it is logical to assume that it decreases over time).
All that is below zero - work not on the TOR + bug fixes (after some growth should also decrease).
Crossing the top line with zero - the planned completion of the work (as if we are working without bugs). But the crossing of both lines - the real completion date.
Make use of it!