You could try something like this in your LOAD script:
SET DateFormat = 'M/D/YYYY'; // or 'D/M/YYYY'??
CROSSTABLE (Milestone, MilestoneDate)
LOAD Project, [Milestone 1], [Milestone 2], [Milestone 3], [Milestone 4], [Milestone 5], [Milestone 6]
Now you have transformed to three fields, Project, Milestone, MilestoneDate.
Now eiter link you milestone dates to a master calendar, create additional date fields in your single table or use a straight table /listbox with a calculated dimension: