At first glance, you may think that a milestone and a due date on any given project are the same things, and I’d even forgive you for thinking so. After all, both have dates that define them; you can’t have either without a date and/or time. But there is a difference.
When plotting out your project on any modern project management software suite, you’ll often have the choice of assigning a task a due date. In addition, you’ll often have a checkbox or a dropdown to set that due date as a milestone.
That said, what’s the difference? And it’s a fair question for those who are learning the ropes. The best answer I’ve seen as of late was found on Reddit:
“All milestones have due dates but not all due dates are milestones.”
A milestone is a due date with significance to the project (or project owner). For example, let’s say your project is working in phases (Phase I, Phase II, etc.); a milestone may represent a specific task that needs to be finished by a stated date (or before) for you to move on to the next phase.
Conversely, just because you have a task with a due date, say, in your Work Breakdown Structure (WBS), doesn’t mean it’s a milestone, per se.
The milestone is also used as a metric for project managers and sponsors. They should be able to glance at the plan and know where the project is in terms of the overall schedule. So, for example, if we needed to be at “milestone 3” two days ago, and we’re only about 75% there, we’ve got issues we need to deal with.
One could use milestones in a project without a date, too. When I worked at Boeing, we had a statistician use milestones on his policy projects. Percentages marked them. However, you won’t see this too often as 99% of projects are based on time and not percentages.