When you assign a task for a concrete Term to an employee, he can really set his own Term taking into account when exactly is it more convenient to perform this task.
For example, you assign a task to perform from Wednesday to Friday, and the employee decides that the most convenient is to perform it exactly Thursday at 3 p.m. and sets this Term.
At this, the employee sees both his Term and the Term you have set. He can not delete the Term you have set. This way it is understood, that the employee must take into account the Term you have set.