Date Calculations - Vb.net

[kenone]

i Have a program in which VB.NET frontEnd and SQL Server2000 at the backend ,that should calculate dates as follows:
RETIREDATE = BIRTHDATE + 60YEARS. BirthDate should be input manually while RETIREDATE should be automatically displayed as a result of adding 60years to BIRTHDATE.The code doesnt work at all as it just keep display a strange RETIREDATE ie 01/01/0001,below is my code ;

Public Overloads Function DateAdd( _
           ByVal Interval As DateInterval, _
           ByVal Number As Integer, _
           ByVal DateValue As DateTime _
        ) As DateTime
        
    End Function

Private Sub txtDOB_Leave(ByVal sender As Object, ByVal e As System.EventArgs) Handles txtDOB.Leave
        Dim rdate As DateTime
        Dim dob As DateTime
 
        rdate = DateAdd(DateInterval.Year, 60, txtDOB.Text)
        txtRetireDate.Text = rdate.ToShortDateString
    End Sub

note: DOB stands for DATEOFBIRTH
txtRetireDate should be automatically displayed after txtDOB.text being lost focus (on leave event).
Is there any thing wrong with my code bove??please help