You can add multiple 'Milestones' to your projects. Firstly select either the 'All' of 'Custom order' View in the 'Tasks' section of your workspace.

Click the arrow on the 'New task' button and select 'New milestone' from the drop down menu. Give your milestone a title and click the calendar icon to add a due date.

The milestone will now appear in your task list and on the Timeline view, it can be moved by dragging and dropping.