Setting the end date

When you have the start date set, it would be logical to open the calendar widget for the end date displaying the month of the start date. If you plan ahead and create activities that start in the next month or later you always need to jump to the correct month.
Focusing on the right month is easy to implement and would improve usability. This is the first thing users do when they evaluate your product, and this small issue may easily put them off.