The question is clear, I don't see what is the difference between "Von etw ab" and "Von etw an", can someone explain the difference ? thanks.
Here's one difference: if you're referring to time, you can only say "von dann an". If you're referring to somebody or something leaving some place, you can only say "von dort ab". Example: "Von Morgen an fährt der Zug vom Hauptbahnhof ab." Starting tomorrow, the train will leave from the central station.