[Tom Bunch]

Milestones have no duration, so they have the same end and start date. So just enter the date you need the milestone to be completed by in either the actual start or actual end text field in the Task: Information inspector.

If you really literally mean "one year after the project start date" and expect it to update if the project start date changes, you'd have to get a little more advanced. Add a "Project start date" milestone and don't add constraints of any kind to it so that it will start "As early as constraints allow". Then add a "Project end date" milestone. Drag a dependency from the start milestone the end milestone, inspect the end milestone in the Task: Dependencies inspector and add 52w lead time. Now if you change the project start date, the start milestone will float to the new date and the end milestone will be 52 weeks later. That's not exactly one calendar year, but the only way I can think of to get that is to mess with your effort conversion factors, and that still won't handle leap years.

-Tom