Adding milestones

Hi

I am adding milestones to my plan and am entering a task with 0 days and then changing the start/finish date to reflect the date of the milestone. I keep getting a smart tag appear telling me this is not the best way to schedule a task.

Should I be entering the due date of the milestone in another way? Have read the project guide but it tells me to enter these via Advanced tab in Task Information Dialogue and says nothing regarding entering the date??

Thanks