There are accessibility issues with the calendar portion of the DatePicker which can be seen in the following demo:

https://demos.telerik.com/aspnet-ajax/datepicker/accessibilityandinternationalization/wcag2.0andsection508accessibility/defaultcs.aspx

• JAWS does not announce the selected date in the Date Grid - While running JAWS, open the calendar and use the arrow keys to move through the date grid. Notice that JAWS does not announce the date numbers, so there's no way to know what date is currently focused. 
• The TAB does not refocus the DateInput back from the popup - While the calendar is open, press the Tab key. Note that focus leaves the calendar. Even if the Tab is not the preferred form of navigation, it should be trapped as long as the calendar widget is open so that it cannot interrupt the user's interaction with the control.
• Escape does not focus the DateInput/Escape does not work in Firefox - While the calendar is open and focused, press the escape key to close it. Note that focus is not returned to the calendar button. It's set top of the page. The calendar button IS focused when a date is selected from the calendar, so it should be possible to do the same when closing the calendar via any other means.
• *Moved to separate feedback item* No direct way to select the prev/next month and year buttons - While the calendar is open and focused, note that there is no way to focus and use the month and year changing buttons in the calendar header.
  • https://feedback.telerik.com/aspnet-ajax/1535165-no-direct-way-to-select-the-prev-next-month-and-year-buttons-when-enablekeyboardnavigation-true 

All of the above were tested in both Chrome, IE, and Firefox.

Combined, these issues seem to break accessibility for both low-vision and keyboard-only users. Any improvements you can make in future releases would be greatly appreciated.