We are missing information. How can both the start date and end date be set if you only know the duration? You will at least have to provide two variables to calculate on the third. X + Y = 25, means that both  X and Y can be anything between 0 and 25. So at least the Start or the End date need to be set, to calculate the other one.

And because of this, does the second requirement make sense? If you have the start or end date and you select another duration, the other field can be calculated as well.