When ran on 1366x768 resolution and the DPI is set to higher value than the default one, in some cases the control is not rendered correctly.
RadCalendar has bigger buttons in Windows8Touch than other themes by design. That's why we have set MinHeign and MinWidh properties to 40 of CalendarButton style to achieve buttons desired size. That is the reason when there is not enough space CalendarButton content to be cut. There is an easy solution setting MinHeight/MinWidht property to a lower value - check the attached demo project.