If you just add a DateTime Picker (from the Windows Phone Toolkit) to your application, it will work but its icons will be very unsatisfactory – just a pair of X’s. To get the proper icons, follow these steps:
1. Add the DateTime Picker to your application
2. Copy the Toolkit.Content folder from the Toolkit Samples to your application
3. Create a Toolkit.Content folder in your application and Add-Existing to add the contents to your application
4. Set all of the .png files from Resource to Content
5. Rebuild and run. Hey Presto! The correct icons appear.