Take a book with you next time, Dragonfly and make sure that you are seen to be reading it when she appears. Comment "I thought I might as well bring it and finish what I was reading, as I knew you are always delayed. Next time we meet, I think I shall put it in my diary as half-an-hour later than we arrange"
With a smile! - but the next time you do arrange something, say "I'll put that down as xxx then - but you be there at yyy!"
She is probably not even aware that she does it, but if it is out in the open she may make a point of leaving a bit earlier - and will get there at the right time. Maybe!