I've read several posts and still cannot address my problem.
I have a fixed duration task, not effort-driven. The standard calendar and default working times are 8h/d and no holidays.
I then create a task, ex: Start = 1/3/11 ; Dur = 21 days and no resources assigned yet. MSP derives a Finish date of 1/31/11, which is appropriate. However, if I then change the default work week to 7.5h/d (37.5h/w), MSP recalculates the Finish to be 1/28/11.
I don't understand why. I expected the duration to remain as 21 days; Finish to remain at 1/31, which is 21 work days, but the work would have calculated out to 157.5h. Help? Or point me to where I need a working time 101?