Milestones represent specific points in time during which an important event has occurred or is scheduled to occur. To add a milestone, click and drag the Line Milestone shape onto a chosen interval on the timeline. It need not be dropped exactly on a specific interval, since you can manually configure the exact date and time of the event.
When you drag the milestone onto the timeline, the Configure Milestone dialog box appears, where you can specify the exact date and time of the milestone along with a description of the event. You can also choose from the standard date formats to better represent the time and date. Click OK to create the milestone on the timeline. The milestone can be dragged along the timeline, and the date and time of the milestone will be updated accordingly.