Timer - start/stop/update

gistfile1.js

var timerID = 0,
    tStart  = null,
    clockID = null;

function UpdateTimer() {
    if (timerID) {
        clearTimeout(timerID);
        clockID = 0;
    }

    if (!tStart) {
        tStart = new Date();
    }

    var tDate = new Date(),
        tDiff = tDate.getTime() - tStart.getTime();

    tDate.setTime(tDiff);
    document.theTimer.theTime.value = tDate.getMinutes() +':'+ tDate.getSeconds();
    timerID = setTimeout('UpdateTimer()', 1000);
}

function Start() {
    tStart = new Date();
    document.theTimer.theTime.value = '00:00';
    timerID = setTimeout('UpdateTimer()', 1000);
}

function Stop() {
    if (timerID) {
        clearTimeout(timerID);
        timerID  = 0;
    }
    tStart = null;
}

function Reset() {
    tStart = null;
    document.theTimer.theTime.value = '00:00';
}