It also depends on the time Project has assumed for the start and end, the
number of hours considered to be working time hours between start and finish
according the the calendar governing the task, and the definition of a "day"
in the Calendar Options settings. Here's how it could work ... If
Tools/Options/Calendar says that "Hours per Day" = 8, the task starts Mon at
12:00 noon and finishes Fri 12:00 noon and the working time calendar says
work hours are 0800-1200 and 1300-1700 - Monday we'd get 4 hours of work,
Tue, Wed and Thur 8 hours each, and Fri 4 hours for a total of 32 hours of
work. 32hours/8hours per day = 4 days of duration. Date fields are really
date/time fields and durations are ALWAYS stored and computed in working
time minutes, regardless of what units are chosen to input or display them
in.
To see what's going on for sure, go to the Tools/Options/View dialog box and
set the date format to one that includes both date and time.