Updated value is not reflected in GUI in WPF

See more:

C#

WPF

I have created one Tab application which contains One main window and related tab, each tab has a separate View-Model.
Each tab initialized one by one, Once the app is open properly then I want to update some values form the second tab and these changes should be reflected on the First tab.
So I have implemented INotifyPropertChanged event related to these properties but it's not reflected on First Tab.
It seems there is some problem in DataContext is not updated properly while changing the value in the second tab.

What I have tried:

I have tried to updated First tab DataContext while changing the values(Property's setter) form the Second tab. I have seen in debugging the property has updated but it's not reflected in GUI.

Thanks.

<pre>//First tab initialization .
public partial class PersonUserControl : UserControl
    {
        /// <summary>
        ///     Default constructor.
        /// </summary>
        public PersonUserControl ()
        {
            InitializeComponent();

            // Allow for UserControl to init successfully within VS Designer.
            if ( !DesignerProperties.GetIsInDesignMode(this) )
            {
                DataContext = new PersonViewModel();
            }
        }

 //First tab View- Model: PersonViewModel.cs

 public string JoinDate
        {
            get
            {
                return _joinDate;
            }
            set
            {
                if (_joinDate != value)
                {
                    _joinDate = value;
                    OnPropertyChanged("JoinDate");
                }
            }
        }

//First Tab XAML component.

<pre> <TextBlock FontWeight="Bold" 
               Height="28" 
               HorizontalAlignment="Left" 
               Margin="30,350,0,0"
               Text="{Binding JoinDate, Mode=OneWay}" 
               VerticalAlignment="Top" />


//Second tab initialization.

public partial class UpdateInfoControl : UserControl
    {
        /// <summary>
        ///     Default constructor.
        /// </summary>
        public UpdateInfoControl()
        {
            InitializeComponent();

            // Allow for UserControl to init successfully within VS Designer.
            if ( !DesignerProperties.GetIsInDesignMode(this) )
            {
                DataContext = new UpdateInfoModel();
            }
        }

//Second tab View-Model: UpdateInfoModel.cs

public DateTime TimeOfDay
        {
            get
            {
                return _timeOfDay;
            }
            set
            {
		if ( _timeOfDay != value )
		  {
		          _timeOfDay = value;
		           OnPropertyChanged("TimeOfDay");
		
			_updateInfoViewModel.JoinDate = timeOfDay .ToString();
		}
		
             }
}

//Second Tab XAML component.

<DatePicker Height="26"
                          HorizontalAlignment="Right"
                          Margin="0,69,239,0"
                          SelectedDate="{Binding TimeOfDay, Mode=TwoWay}"
                          SelectedDateFormat="Long"
                          VerticalAlignment="Top"
                          Width="208" />