Re: DatePicker messing up!
The WPF forum is over here. I will ask a mod to move it for you as this question directly relates a specific WPF control and not a control from Windows Forms.
Re: DatePicker messing up!
DisplayDate is a confusing property... I think it's for situations where you want to preserve the user's selection but possibly display another date? Either way, its default value is Today (per documentation) and it isn't what you want. Use the SelectedDate property instead. This is spelled out clearly in the example in the documentation.
Re: DatePicker messing up!