You can enter durations in whole days but you need to be aware that in all
cases Project actually records and calculates durations in minutes to the
nearest 10th. If you use any other units, Project converts them on entry
and for display and you really don't have any option to that. Assuming your
setting in Tools, Options, Calendar, Hours per Day is set to the default 8
hours, a "day" is defined as any time interval of 480.0 minutes and when you
enter a task as having a duration of "2 days" Project will convert that to
960.0 minutes prior to storing it. Let's say that task starts Monday at
8am. The working time calendar tells Project what minutes out of the day
will serve to burn up duration and what minutes don't count. If your
calendar says work takes place between 8am and 5pm with an hour for lunch,
that 2 day task will end Tuesday at 5pm, 960.0 calendar working minutes
later. If you change your working time calendar to show hours of work as
8am to 3pm with a 30 minute lunch, the task will end Wednesday at 11am,
again 960.0 working time minutes after it starts. But if you assign it to a
part time person who works 4 hours per day, 1pm to 5 pm, that same 2 day
task that originally was starting Monday at 8am will reschedule itself to
start Monday at 1pm (when the resource gets there controls when it can
start) and will end 960.0 of the RESOURCE'S working minutes later, Thursday
at 5pm. The duration is exactly the same in all three cases - 2 days - but
the calendar dates and times for start and finish you see for each scenario
will be wildly different.
Remember that a task always produces a tangible output and that takes an
exact amount of time to produce. The amount of output doesn't expand or
contract based on the time allowed for the resource to do it. Just the
reverse - the time expands or contracts so it always matches the time it
takes to produce the required output, no more and no less. Resource's
produce output at a fixed rate (widgets per hour, per minute, etc) and a
task requires an exact amount of output, 100 widgets no more and no less,
and so required output controls the time it will take, not the other way
around.