I am running Framework version 18.104.22.168, and I do not have AllowMultipleSelection enabled.
This is actually a very simple test:
1) Create a new C# Mobile Web Application project (from the 51Degrees template)
2) Drop a Calendar control and a Label on the default form from 51Degrees.mobi in the Toolbox
3) Add a SelectionChanged event handler
4) Put the following line of code in the handler:
Label1.Text = Calendar1.SelectedDate.ToShortDateString();
Run the project and select dates. I see the label not properly reflecting the currently selected date, but rather the previous one.