In admin2, the calendar popup accessed in date/time fields doesn't take the window size in consideration for display purposes. It always open downwards regardless of whether it fits or not.

Steps to reproduce

Resize your browser window so that an editing date/time field is close to the bottom of it
Click the calendar button and watch that you'll need to scroll down to see the calendar, despite the fact that you (might) have a lot of room for the popup above