A Milestone is a board with a due date.
A Milestone can be set via a board's dropdown menu. Select the "Edit" option. When you click on the Milestone Date entry field, you will be presented with a calendar function with a date and time selector. Just input the due date and select Save (or Remove).
If a milestone has been set on a board, a date will appear next to the dropdown menu arrrow , as well as a clock icon, indicating there is a time associated as well.