To reiterate what Jan said - you should NOT enter start or end dates for
tasks except under the very specific circumstances where you need to
establish contraints. And if you do, you can't enter both - the results
you get if you try depend on the order you enter them and the links coming
into the task in question.
Several entries in Project interact. First, on the Tools Options Menu page
there is the "hours per day" field. When you enter a duration of X days it
controls how many hours that converts to. Then there's the Project Calendar
which determines how many hours there are between, say, Monday at 8am and
Friday at 5pm, that are counted as duration hours. Whatever the working
hours as defined by your Project Calendar are between your start and end
dates & times (and all dates are actually date/time fields), converted to
the units of "day" as governed by the "hours per day" setting, is what is
going to be displayed in the duration field. Further interacting, if you
typed in those start and end dates, the entries on the Option page for
"default start time" and "default end time" are the times at which project
will schedule the task's start or end constraints.
