I recently ran across an online Project Planning application that had a very handy feature I haven't seen before. When you click to enter either a Start Date or Due Date, it opens the same dialog box with two calendars, one for each. Don't have to click to open, enter, close, click to open, enter, close - it saves the two steps of closing and re-opening in the middle of the process. Of course, you could enter only one if you prefer, with no penalty in ease of use or clicks.
Since the Start Date and Due Date dialog boxes need some fixing anyway, why not just combine them as well?