cant call code behind function from javascript in asp.net

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C#

ASP.NET

Javascript

<script type="text/javascript" language ="javascript" >
        var tim;

        var min = 1;
        var sec = 60;
        var f = new Date();
        function f1() {
            f2();
            document.getElementById("starttime").innerHTML = "Your started your Exam at " + f.getHours() + ":" + f.getMinutes();

            document.getElementById("endtime").innerHTML = "Your  time is :" + f.toLocaleTimeString();
        }
        function f2() {
            if (parseInt(sec) > 0) {
                sec = parseInt(sec) - 1;
                document.getElementById("showtime").innerHTML = "Your Left Time  is :" + min + " Minutes ," + sec + " Seconds";
                tim = setTimeout("f2()", 1000);
            }
            else {
                if (parseInt(sec) == 0) {
                    min = parseInt(min) - 1;
                    if (parseInt(min) == 0) {
                        clearTimeout(tim);

                        document.getElementById('mybutton').click();
                        alert("calling");
                    }
                    else {
                        sec = 60;
                        document.getElementById("showtime").innerHTML = "Your Left Time  is :" + min + " Minutes ," + sec + " Seconds";
                        tim = setTimeout("f2()", 1000);
                    }
                }

            }
        }
    </script>

my whole code is in update panel.
and this is a modal popup window

this is my javascript code....mybtn is the button its inside panel. I want to call its click event or any public void function when timer stops.