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)

i like to post an update to my post above, which makes it more complicated...

above I said activity BB is a future activity but marked as 100% by giving future actual start and finish dates. (although this can not happen, i know, but my schedule was not updated for few weeks and i knew now that a few days ago that activity started and then finished, so i marked it that way)

now i updated my data date. and now activity BB dates are past.

but still the same thing happens. when i make activty B a predecessor of activity AA, and enter actual start and finish dates which are different to AA, duration of 0 is calculated to AA by the program

There is a checkbox in Task information that allows you to mark a non-zero day duration task as a milestone. Is that what you mean by "a milestone (activity with zero duration) with different start and finish dates"

I'm afraid I don't understand what you are asking. How many tasks are we talking about - I see BB, AA, B. Can you please try again? Project will pay attention to link relationships and attempt to move successor task progress where it's logical. Please attach a file or a picture of what you're talking about.