Calendar Event ending Midnight

I would totally agree with you but Calendar currently reads the input 2014-04-14 00:00 as 2014-04-13 24:00 and resets the beginning to 2014-04-13 23:00. So Calendar goes a day back versus a day ahead and now thinks you put the start after the end and resets it. So I think that is a bug in the time picker. They should add 24:00 and then convert it to 00:00 the next day.

Hm?

Just tried adding an event

Test
start 2014-04-15 00:00
end 2014-04-16 04:26

and got an event lasting from 2014-04-15 00:00 to 2014-04-16 04:26 (as expected).

If you go to Settings → System → Language and time: Time and date, is "Automatic update" enabled (mine is)?

(just read through "Time zone changes to Finland, Helsinki when enabling "Automatic update" while no SIM card is inserted" - do you still have issues with your SIM-card (I saw your answer over at "Sim card connection to phone lost easily") - could that be part of the issue?)

So: I tried recreating what you actually seem to do:

I edited above event to

start 2014-04-15 13:00
end 2014-04-15 00:00 (i.e. same date, but before the start time)

which ended up (even before "saving" it) as an event lasting from 2014-04-14 23:00 to 2014-04-15 00:00.

... or have I misunderstood your start- and end-times?

No the second attempt was completely right. I redid it yesterday again, just before 23:00. This triggered the bug right away without me entering anything. Presets were start 23:00 end 00:00 but the event ended up being the day before the day I picked. Personally I think they should add 24:00 to the time picker. So if you come from the right you get 00:00 and if you come from the left you get 24:00, which actually is 00:00 the next day.