I found a way to make a milestone (activity with zero duration) with different start and finish dates. but don't know why it is happening and that is my question.

normally, when you enter an actual start and finish date to an activity, lets say activity AA, MS project calculates a duration based on the difference.



but, if that activity AA, has a predecessor BB, such as BB is a 100% item and it contains future dates, then, even if you give actual start and finish dates different from each other to activity AA, MS project makes it a zero duration activity.

Why is this happening?

Also, can this be a solution to the problem of excluding an item from % calculation? (as we can give it a zero duration this way, but still show start and finish dates)