Select a date from RadDateTimePicker using the calendar dropdown. Now type another date in the textbox to make a new selection. Open the drop-down and you will see that there are two dates 'selected'. The issue occurs, because when you select from the calendar, you set the cell to Focused.Selected. When you then type a date in the textbox, you change the Focused cell, but the selection on the previous cell remains. So, you end up with two 'selected' cells. The workaround is to clear the selection on opening the dropdown: 

        void radDateTimePicker1_Opening(object sender, CancelEventArgs e)
        {
            calendar.SelectedDates.Clear();
        }