Duration values for single milestones

Question:

Why does a single milestone automatically have a 0-day duration instead of 1 day?

Answer:

If Schedule Details does not allow Hourly or Minute detail, then a single Milestone will show 0 duration. However, for a milestone to show 1-day duration, or a specific hourly duration, Schedule Details must allow at least Hourly detail.

Choose More Settings in the Start and End Dates section of the Dates tab.
Choose Hourly/Minute...
Allow Hourly Detail and Allow Minute Detail.