Late dropping plums?

[homegrownbucks]

Anyone know of a zone 4 hardy plum variety that drops during sept or oct?

 

[Turkey Creek]

I am growing 4 or 5 varieties that are reportedly hardy to -30 degrees. I just started growing them last year so I do not have any history with fruit drop. A couple of them ares supposed to ripen late August and first week of September in central MN.