Why this error message shown :string was not recognized as a valid datetime

Private Function CalculateAge(ByVal Dob As DateTime) As String
       Dim Now As DateTime = DateTime.Now
       Dim Years As Integer = New DateTime(DateTime.Now.Subtract(Dob).Ticks).Year - 1
       Dim PastYearDate As DateTime = Dob.AddYears(Years)
       Dim Months As Integer = 0
       For i As Integer = 1 To 12
           If PastYearDate.AddMonths(i) = Now Then
               Months = i
               Exit For
           ElseIf PastYearDate.AddMonths(i) >= Now Then
               Months = i - 1
               Exit For
           End If
       Next
       Dim Days As Integer = Now.Subtract(PastYearDate.AddMonths(Months)).Days
       Dim Hours As Integer = Now.Subtract(PastYearDate).Hours
       Dim Minutes As Integer = Now.Subtract(PastYearDate).Minutes
       Dim Seconds As Integer = Now.Subtract(PastYearDate).Seconds
       Return [String].Format("{0} Years", Years)
   End Function


   Protected Sub btnGetAge_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnGetAge.Click
       If (CType(Session.Item("uno"), String) = "") Or (CType(Session.Item("urights"), String) = "") Then
           Server.Transfer("~/default.aspx", True)
       Else
           Dim dtVal As String = Trim(TxtDob.Text)
           Dim Dob As DateTime = Convert.ToDateTime(dtVal)
           'DateTime dob = DateTime.ParseExact(d, "dd/MM/yyyy HH:mm:ss", CultureInfo.InvariantCulture);
           txtage.Text = CalculateAge(Dob)
       End If
   End Sub