validation not working on date picker control

Question

 i am trying to do validation between date1 and date2 for this i am using date picker controls binding properties are string the problem is validation is not working on it, since numeric validation is working fine.

 

i am using

Implements INotifyPropertyChanged 

and under setter using

Public Property EndDate() As String
            Get
                Return _endDate
            End Get
            Set(ByVal value As String)
                If StartDate > value Then
                    Throw New ValidationException("Error")
                End If
                _endDate = value
                RaisePropertyChanged("EndDate")
            End Set
        End Property

 Can u please tell me what is the issue

 

Thanks