How to start setTimeout()

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Javascript

I wants to buils javascript timer. when I start it first time it works fine but when i stops it I can't start it second time, please tell me what mistake I done in my code.
java script code:

var hrs
var mins
var secs;

var date = new Date();
function createCookie(name, value) {
    var expires;
    var days = 1;
    if (days) {
        //var date = new Date();
        date.setTime(date.getTime() + (days * 24 * 60 * 60 * 1000));
        expires = "; expires=" + date.toGMTString();
    }
    else {
        expires = "";
    }
    document.cookie = name + "=" + value + expires + "; path=/";
}

function getCookie(c_name) {
    if (document.cookie.length > 0) {
        c_start = document.cookie.indexOf(c_name + "=");
        if (c_start != -1) {
            c_start = c_start + c_name.length + 1;
            c_end = document.cookie.indexOf(";", c_start);
            if (c_end == -1) {
                c_end = document.cookie.length;
            }
            return unescape(document.cookie.substring(c_start, c_end));
        }
    }
    return "";
}

function cd() {
    //var strTime = document.Form1.disp.value;
    var strTime = getCookie("zz");
    tme = strTime.split(":");
    hrs = 0 * h(tme[0]);
    mins = 0 * m(":" + tme[1]); // change minutes here
    secs = 0 + s(":" + tme[2]); // change seconds here (always add an additional second to your total)
    redo();
}

function h(obj) {
    for (var i = 0; i < obj.length; i++) {
        if (obj.substring(i, i + 1) == ":")
            break;
    }
    return (obj.substring(0, i));
}

function m(obj) {
    for (var i = 0; i < obj.length; i++) {
        if (obj.substring(i, i + 1) == ":")
            break;
    }
    return (obj.substring(i + 1, obj.length));
}

function s(obj) {
    for (var i = 0; i < obj.length; i++) {
        if (obj.substring(i, i + 1) == ":")
            break;
    }
    return (obj.substring(i + 1, obj.length));
}

function dis(hrs, mins, secs) {
    var disp;

    if (hrs <= 9) {
        disp = " 0";
    } else {
        disp = " ";
    }
    disp += hrs + ":";

    if (mins <= 9) {
        disp += "0";
    } else {
        disp += "";
    }
    disp += mins + ":";

    if (secs <= 9) {
        disp += "0" + secs;
    } else {
        disp += secs;
    }
    return (disp);
}

var tm;
function redo() {
    secs++;
    if (secs == 60) {
        secs = 00;
        mins++;
    }
    if (mins == 60) {
        mins = 00;
        secs = 00;
        hrs++;
    }


    document.Form1.disp.value = dis(hrs, mins, secs); // setup additional displays here.
    createCookie("zz", dis(hrs, mins, secs));
    cd = setTimeout("redo()", 1000);

}
function startTime() {
    cd();

}
function stopTime() {
    clearTimeout(cd);
}
function resetTime() {
    createCookie("zz", "00:00:00");
}
function getTime() {
    alert(getCookie("zz"));
}

function init() {
    document.Form1.disp.value = getCookie("zz");
}
window.onload = init;

html code
Time Consumed
<input name="disp" value=" 00:00:00" readonly="readonly" id="disp" class="time_text" type="text"
style="border-style:none"/>

<input id="Button1" type="button" value="start" />
<input id="Button2" type="button" value="stop" />
<input id="Button3" type="button" value="reset" />
<input id="Button4" type="button" value="get" />