catch error Addsecound on type string not found

gistfile1.txt

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
  
        If Not IsPostBack Then
            RadGrid2.DataSource = Data()
            RadGrid2.DataBind()
            For i As Integer = 0 To RadGrid2.Items.Count - 1
                Dim lbltime As Label = RadGrid2.Items.Item(i).FindControl("lblTime")
                StartTime = DateTime.Now.AddMinutes(lbltime.Text).ToString("HH:mm:ss")
                CurrentTime = StartTime.ToString
                lbltime.Text = CurrentTime.ToString("HH:mm:ss").ToString
                Session("time") = lbltime.Text
            Next
        End If
    End Sub

gistfile2.txt

Protected Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
        Try
            For i As Integer = 0 To RadGrid2.Items.Count - 1
                Dim lbltime As Label = RadGrid2.Items.Item(i).FindControl("lblTime")
                ' Dim lbltime1 As Label = RadGrid2.Items.Item(i).FindControl("label3")
                If Val(lbltime.Text) > 0 Then
                    
                    CurrentTime = Session("time").AddSeconds(-1).ToString

                    lbltime.Text = CurrentTime.ToString("HH:mm:ss").ToString
                End If
            Next
            RadGrid2.Rebind()
        Catch ex As Exception 'error Addsecound on type String not found

        End Try
    End Sub