I have found that when scrolling through up and down the years that I get incorrect values. I have attached pictures of these values. Obviously, when I select 1990 I expect to get years listed between 1990 and 2000 but I don't get that.
As per your request, I'm providing a small sample project with the extracted default RadCalendar style for the Office_Black theme applied to the RadDatePicker. Please note that if you're using another theme, you should edit its style accordingly.
Please also note that this project uses the implicit styles approach for styling the controls and if you're using the StyleManager you will need to copy all the required resources from the theme's Telerik.Windows.Controls.Input.xaml file.
Please let me know if using the style from this project solves your issue. If that is not the case, please let me know how I can reproduce the issue you described with the project I provided so that I can further investigate and assist you.
Get quickly onboarded and successful with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers.Learn More.