It depends on the duration of the task and the start time. On the view
page of the Options menu select a format that shows both date and time.
Look at both your project start time and the task start times. If your
work day begins at 8am and goes till 5 and a 1 day task starts at 9 on
Monday, that 1-day duration will end at 9am on Tuesday. As you adjust
durations and resources it's entirely possible that tasks will start and
end at other than concurrent with whole day start and end times.
Remember that regardless of what units you enter duration in, it's
converted to minutes for storage, calculations, and placement in the
charts. Since you say the calendar bar extends into Tuesday by a hair,
that's what I'd suspect is going on. A task with 480 minutes, ie 8
hours, is a 1-day task regardless of how many calendar days it spans in
the calendar. Or conversely, a task that begins at 1pm on Monday and
ends at 12 noon on Tuesday is a 1-day task, not a 2-day'er.