When we hover a whole-day event (single day or day-range) a tooltip appears that shows the day after the single (or last) day as the end date. From a technical point of view, this might be correct, since the time delta for a single day event is 1 day and not 0 days, however this is not what most humans would expect. 
Is there a chance to fix this?
Yes, this has now been fixed.
This is how it appeared in version 8:
And this is how it appears in version 9:
Thanks, Gary, for your quick positive response!
(To be honest, Iām slightly disappointed that it seems, we have to pay for bug fixes, not just improvements.)