Calendar hours and days

Hi, Using the calendar module works fine. But the result is not always as aspected;

Using interval : 1 days -> all meetings from this moment till end of the day?
Wenn using interval : 24 hours -> doesn’t give me the next day meetings
Using : 30 hours -> gives the meetings for the next day

This is not as aspected. The manual is not clear about this. Anybody have a good explanation?