Found the same problem. Tried messing with show times but still had the same problem. And when you have show times on you can't change the length of an entry.
Something definitely not quite right there..
Odd that I've not been able to find anyone else complaining about this. Makes me think that it's me (and Jeff) doing something wrong?
(Obligatory xkcd: Wisdom of the Ancients)
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I got the answer from Oracle support.
Below is what they told me.
APEX calendar leverages the jQuery FullCalendar Plugin. That Plugin treats end dates - by design - as exclusive.
When Show Time is set, then the end date is exclusive at the 'second' level, so for the calendar it ends the second before (which does not make a difference for display). If Show Time is not set, it's exclusive at the 'day' level - which makes a difference for display.
So the bug is actually in our Hover Text (tooltip) it will be fixed next release.
So there are two workarounds
1) set "Show Time" to Yes (and maybe disable day and week views)
2) set Show Time" to No, add one day to the end date in the SQL query and (for now) disable tooltips.