Combined Constraints

Both Project.net and Microsoft Project allow the scheduled dates of a task to be constrained so its starts or finishes no later than, no earlier than, or on a specific date.

Task Constraints

Tasks in Project.net have constraints that require that the start and/or finish date of a task be no later than, no earlier than, or on a specific date.  Constraints are not enforced at all for manual tasks, and for automatic tasks, they are only enforced when the workplan is rescheduled.  This article talks about how constraints can be set, and how they can be visually identified in a workplan.