Please see demo: https://demos.telerik.com/aspnet-ajax/monthyearpicker/accessibility-and-internationalization/wcag-2.0-and-section-508-accessibility/defaultcs.aspx
1. tab to calendar button
2. hit enter to open calendar dialog
3. press tab
Notice that instead of causing the focus to move within the calendar pop-up, focus first shifts through all remaining elements of the underlying page. This make this control difficult for users relying solely on keyboard navigation.
Ideally this control should function more like the standard calendar controls, obtaining focus when opened and returning focus to the date field when closed.