trying to select a date for an event displays both a drop down and a calendar

If I go to add a new event via the upcoming events block, when I click the day number field for the date I get both a drop down of numbers and a popup calendar displayed over the top of one another. Only one or the other should be displayed. I tried to get a screenshot however taking a screenshot causes the drop down box to disappear.

Happens in both firefox and chrome under Ubuntu.

It also occurs in 2.1 stable, 2.2 stable and master (2.3).