Either activating or deactivating plug-in removes the widget from the widget area. Once activated I added the calendar widget back to the widget area. Either with or without a title and selecting all categories and selecting the include future posts checkbox and saving the widget the calendar does not display any posts and does not display the monthly directional arrows. Effectively making the widget show the month name and the days of the month. Once I deactivated the plug-in and moved the calendar back to the widget area everything returned to normal.
The theme is Twenty Ten Version 1.5.
I hope you have a quick fix, as I like the idea of showing future posts for a daily historical calendar in the widget.