public class MainActivity : Activity
{
    DatePicker mDatePicker;
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);

        // Set our view from the "main" layout resource
        SetContentView (Resource.Layout.Main);
        mDatePicker = FindViewById<DatePicker>(Resource.Id.mDatePicker);
    }

    public override bool OnKeyUp([GeneratedEnum] Keycode keyCode, KeyEvent e)
    {
        //when user press c, the date of the DatePicker will be updated
        if (keyCode == Keycode.C)
        {
            mDatePicker.UpdateDate(mDatePicker.Year, mDatePicker.Month, mDatePicker.DayOfMonth + 1);
        }
        return true;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
  <DatePicker
    android:id="@+id/mDatePicker"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"/>
</LinearLayout>