when you put the duration in, it shows as "2 days", "3 days", or whatever the duration is according to the calender being used (as you have found out!). Fortunately there is a way of ignoring the calendars, by using edays (elapsed, or extended days, depending on which books you read). This ignores the calender and uses real days. To set your task to 28 actual days, under duration type "28 edays", and this will give the required result.

The other way to do it would be to set up a calender that ignores all holidays and includes weekends, but for your purpose, I would think that edays would solve the problem in a much easier way.