Source: JOPX on SharePoint 2010, SharePoint 2007 and Windows Phone 7
To see the correct ApplicationBar icons in the DatePicker and TimePicker, you will need to create a folder in the root of your project called "Toolkit.Content" and put the icons in there. The toolkit provides the necessary icons, but you have to copy them from the PhoneToolkitSample project. They must be named "ApplicationBar.Cancel.png" and "ApplicationBar.Check.png" and the build action must be "Content"!
Source: WP7 DatePicker and TimePicker in depth | API and Customization
Joris Poelmans, one of our experts, regularly blogs at JOPX on SharePoint 2010, SharePoint 2007 and Windows Phone 7.
