Thank you both for your replies. However they do not solve the problem I (at least tried to) describe.
I completely agree with you that the events that are 24h+ should not have the allday option set. The problem is that, regardless how it is handled, the end date goes from being inclusive to being exclusive if the allday option is disabled. This is rather confusing for a user that have learned to expect inclusive treatment of end dates. Having this conception of end dates, the month view seems inconsistent when working with the dates of mixed allday and non-allday events, as the same kind of visualization of event "ends" does not result in the same date.
This behavior does not go away by treating the allday option as you described. Therefore I am asking for a way to treat non-allday events end dates in an inclusive manner. Like this, if I describe it with the correct allday option configuration:
Allday event: 20th august - 20th august (this is the whole day on the 20th of august)
Non-allday event: 19th august 00:00 - 20th august 23:59 (this is spanning two whole days, 19th - 20th)
In this way, the dates are always consistent regardless of what the allday option is set to. In the month view (and allday section) this means that an event that visually end on the 20th, indeed ends on the 20th. Current behavior the non-allday event would end on the 21st in this case.
Thanks beforehand for your reply.