When trying to draw Gantt diagram for long projects, the scale is not good enough. It could be useful to be able to set the "resolution" of the gantt (i.e. month or year resolution).
to improve the readability in that case, milestones should also write exact date.
example for syntax proposal:
Resolution is Month
[Test prototype] lasts 10 days
[Prototype completed] happens at [Test prototype]'s end
[Prototype completed] display date
[Setup assembly line] lasts 12 days
[Setup assembly line] starts at [Test prototype]'s end