passing an html id into javascript function

I noticed a few other problems with your code, I've commented the fixes and added a couple of other tips too.

var seconds = 0;
var clockId;
var correctAns;

// Lets get a reference to the quizclock element and save it in
// a variable named quizclock
var quizclock = document.getElementById('quizclock');

function runClock() {
    // seconds + 1;
    // This calculates seconds + 1 and then throws it away,
    // you need to save it back in to the variable
    // You could do that with:
    // seconds = seconds + 1;
    // But it would be even better with the shorthand:
    seconds += 1;

    // set the HTML inside of the quizclock element to new time
    quizclock.innerHTML = seconds;
}

function startClock() {
    showQuiz();
    runClock();

    // setInterval("runClock()", 1000);
    // When using setInterval and setTimeout you generally just
    // want to directly pass it the function by name. Passing it
    // a string "runClock()" is in effect actually running
    // eval("runClock()"), eval should be avoided unless you
    // really need it.
    // setInterval returns a number which identifies the interval,
    // you need to save that number, you'll need it when you
    // call clearInterval
    clockId = setInterval(runClock, 1000);
}

function stopClock() {
    // clearInterval takes the id that setInterval
        // returned to clear the interval
    clearInterval(clockId);

    gradeQuiz();

    // you had this alert statment after the return statement,
    // it would have never run, return statements end the
    // function and anything after them is ignored
    alert("You have " + correctAns + " correct out of 5 in " +
        quizclock + " seconds.");

    //return = correctAns;
    // the return statement doesn't need a =,
    // return = correctAns says set a variable named return to the
    // value of correctAns since return is a reserved word,
    // that should generate an error
    return correctAns;
}

Some useful reference links:

• setInterval
• clearInterval
• getElementById
• Reserved Words (Things that can't be used as variable names)
• Assignment Operators (More shortcut operators listed here)
• Introducing the JavaScript DOM
• An Inconvenient API: The Theory of the Dom

If this is for a formal class you might have to just use basic DOM methods to get elements (getElementById, etc). If you are just learning on your own I would encourage you to learn a DOM library. I would suggest jQuery, it is easy to learn and is now more or less the de facto standard. With jQuery instead of document.getElementById('quizclock') you could just do this: $('#quizclock'). Using jQuery makes your code a little shorter, standardizes things between different browsers and helps protect you from bugs in those browsers.

You are just a beginner now, in small examples like this you don't need to worry about global variables, but you should know that it is generally a bad idea to use too many of them. What if another function on the page also used a global variable named seconds? It might change seconds and screw up your timer. This is getting a little advance, but one way to avoid this is to wrap your code in a self-invoking anonymous function:

(function () {
    var seconds = 0;

    // inside here seconds is visible and can be used

}());

// outside seconds is not declared, it will return undefined.

Unfortunately any functions inside will also not be visible on the outside, so attaching them via onclick= wouldn't work but you could (should) attach them in using the DOM:

var submitButton = document.getElementById('submitanswers'); // you'll have to give the button an id
submitButton.addEventListener('click', stopClock, false);

Again, using jQuery would make this even easier:

$('#submitanswers').on('click', stopClock);

Likewise if you use jQuery, it already forces you to wrap your code in a function which will keep your variables out of the globalnamespace:

$(document).ready(function () {
    var seconds;
    // again seconds is visible here
});

// but not here

You can select an element with:

var quizclock = document.getElementById('quizclock');

You can then set the value with:

quizclock.innerHTML = seconds;