If you are using 24h calendar, that is 24 working hours per day. When you enter 28d as duration of some task it means 28x8 hours, no matter what calendar you are using. If the task you are entering lasts for 28x24 hours than you have to enter something like 28wd (w means whole). I am not sure if the letter 'w' will be correct for your version of MS Project. I am using Czech version of Project and there is letter 'u'. 'w' could be in english version probably...