You have the option of defining a 'Deadline' on the milestones.
The deadline is an objective that is set manually (for instance a date defined by the sponsor of the project).
The scheduling engine won’t move it. It is fixed. The scheduling process will only impact the finish date. You have probably noticed that the deadline is displayed on the gantt view. If you are still delivering before the deadline, it will be displayed in green. If you are missing the deadline (positioned before the milestone in the gantt view), it will be displayed in red.