WPF DatePicker default to today's date

please try with this ....

<my:DatePicker SelectedDate="{x:Static sys:DateTime.Now}"/>

add this reference

xmlns:sys="clr-namespace:System;assembly=mscorlib"

The below works for me. (dpDate is my DatePicker control)

public MainWindow()
{
    InitializeComponent();
    dpDate.SelectedDate = DateTime.Today;            
}

Couple of options...

You could copy the entire style for the DatePicker control and edit the XAML and use that as the default resource making the TargetType DatePicker which will force it to be used across the application.

You could also edit the style locally and place your own TextBox in the style and hide the DatePickerTextBox and then setup the binding appropriately.

Some in depth conversation here with a pretty good explanation by ericf at the top.