Timer and Update Panel

Question

I have a page in my asp.net web forms with a timer and an update panel. My page is on a Master page. 

 

<asp:Content ID="Content1" ContentPlaceHolderID="MainContent" runat="server">
    <br />
    <asp:Timer ID="Timer1" runat="server"
        Interval="12000" OnTick="Timer1_Tick">
    </asp:Timer>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
        <ContentTemplate>
            <fieldset class="container">
                <legend><strong>Formal Teacher Evaluation</strong></legend>

                <div class="row">
                    <div class="col-lg-2">
                        <asp:Label ID="label1" runat="server"></asp:Label>
                    </div>
                </div>
            </fieldset>
        </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="Timer1" EventName="Tick" />
        </Triggers>
    </asp:UpdatePanel>
</asp:Content>

 in code behind I have this:

 

protected void Timer1_Tick(object sender, EventArgs e)
        {
            label1.Text = "Time updated : " + time;
        }

It updates after the interval time lapsed. But, the panel only updates once. How can I make the update panel updates every after interval time? 

 

Thank you for any help.