Dates on events are for some dates interpreted wrong. I created this event in May, but it shows up in October:

When I view date fields in Notes document properties I see the following values:
StartDate: 2013-05-10, StartDateTime: 05.10.2013 00:00:00 CEDT
EndDate: 2013-05-12, EndDateTime: 05.12.2013 00:00:00 CET
This is running on a Domino 8.5.3 server, with Norwegian Language Pack and date settings