Binding 3 textboxes together; same DateTime differ

I have three textboxes right now:

<TextBox Name="SDate1"  Text="{Binding Source={x:Static System:DateTime.Now}, Mode=OneWay, StringFormat='MM/dd/yyyy'}"  />
<TextBox Name="SDate2" />
<TextBox Name="STime1" Text="{Binding Source={x:Static System:DateTime.Now}, Mode=OneWay, StringFormat='hh:mm:ss'}" />

In the code behind:

SDate2.Text = String.Format("{0},{1}/{2}",
           now.Year,
           now.DayOfYear.ToString("d3"),
           now.ToString("HHmmss"));

What this looks like:

What I would like to do is, if I edit any part of one textbox, it should edit the others as well at the appropriate part. So If I changed the 2016 part in the first text box to 2017, the SDate2 like so:

This should also work if I change the hours/minutes/seconds in STime which will change the last part of SDate2 and vice versa.

Changing the day in SDate1 will change the dayOfYear in SDate2 and vice versa.

What would be the best way to achieve something like this?

Edit: I can bind the textboxes together, but the format isn't kept, and it's the same text in all the textboxes.

Edit 2: Here's the code behind in the xaml.cs file I tried.

public partial class TestDate : Window, INotifyPropertyChanged
{      
    private DateTime _dateInViewModel;

    public TestDate()
    {
        InitializeComponent();
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, e);
        }
    }
    public DateTime DateInViewModel
    {

        get { return _dateInViewModel; }
        set
        {
           _dateInViewModel = value;
           OnPropertyChanged(new PropertyChangedEventArgs("DateInViewModel"));
        }
    }
}

Here's the update XAML for the textboxes:

<TextBox Name="SDate1" Text="{Binding DateInViewModel, StringFormat='MM/dd/yyyy'}" />
<TextBox Name="SDate2" Text="{Binding DateInViewModel}" />
<TextBox Name="STime1" Text="{Binding DateInViewModel, StringFormat='hh:mm:ss'}" />

The reason this is not working is because you are binding to separate DateTime instances. All three TextBoxes should bind to the same DateTime instance. In a proper MVVM implementation you should have a viewmodel with a DateTime property, lets call it DateInViewModel:

private DateTime _dateInViewModel;
public DateTime DateInViewModel
{
    get { return _dateInViewModel; }
    set
    {
        _dateInViewModel = value;
        NotifyPropertyChanged("DateInViewModel");
    }
}

The viewmodel should implement the INotifyPropertyChanged interface, so the bindings are notified when the property is changing.

Instead of setting SDate2's Text to that string.Format(...) you just set DateInViewModel in the viewmodel:

DateInViewModel = DateTime.Now;

And finally your XAML:

<TextBox Name="SDate1" Text="{Binding DateInViewModel, StringFormat='MM/dd/yyyy'}" />
<TextBox Name="SDate2" Text="{Binding DateInViewModel}" />
<TextBox Name="STime1" Text="{Binding DateInViewModel, StringFormat='hh:mm:ss'}" />

You can change the bindings' UpdateSourceTrigger to PropertyChanged to see instant updates as you edit. The default UpdateSourceTrigger for the TextBox's Text binding is LostFocus so other bindings are only notified when you click outside the TextBox.