P6 - Additional 1 day duration

In the programme I'm working using P6, I'm using "day" as the unit of time. I set Admin Preferences>Time periods to 24 Hours/Day. I did the same for the calendar. But for an activity with a 1 day duration, which will start on 08-Aug-11 00:00, it will be finished on 09-Aug-11 00:00. Actually it is the same for every activity of any duration. So if I will not show the time, if you look at the start and finis date, it is like having an additional 1-day for the duration. Anybody knows how to fixed this? I read some previous thread but I haven't found any solution. Thanks.