I post an event in Google Calendar, say for example from 12:00 am - 1:00 am. I tried to extent that particular event to the whole day till 11:30 pm, but I couldn't extend it more than 11:00 pm. Is there any specific reason for that?
It's probably because, if you wanted a whole-day event, you can create this sort of thing by clicking in the zone just under the day's date.
Otherwise you can create your event, click it, select "edit event" and change the end time to whatever you like.
I do believe you've found a small bug related to how the drag extender works.
However, this may be as designed to prevent display weirdness from happening since events longer than 24 hours automatically go into the all-day event area of the calendar regardless of being marked as all-day.
Here's what I found: