怎么使用js在网页计时

function startCountdown(duration, display) {
    let timer = duration, minutes, seconds;
    const interval = setInterval(function () {
        minutes = parseInt(timer / 60, 10);
        seconds = parseInt(timer % 60, 10);

        minutes = minutes < 10 ? "0" + minutes : minutes;
        seconds = seconds < 10 ? "0" + seconds : seconds;

        display.textContent = minutes + ":" + seconds;

        if (--timer < 0) {
            clearInterval(interval);
            // 倒计时结束后的操作
        }
    }, 1000);
}

window.onload = function () {
    const display = document.querySelector('#time');
    startCountdown(60, display); // 开始一个60秒的倒计时
};

function startTime() {
    const today = new Date();
    let h = today.getHours();
    let m = today.getMinutes();
    let s = today.getSeconds();
    m = checkTime(m);
    s = checkTime(s);
    document.getElementById('clock').innerHTML = h + ":" + m + ":" + s;
    setTimeout(startTime, 500);
}

function checkTime(i) {
    if (i < 10) {i = "0" + i};  // add zero in front of numbers < 10
    return i;
}

window.onload = startTime;