一、实例代码

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Jquery实现定时器案例</title>
    <style>
        #demo {
            width: 300px;
            margin: 10% auto;
        }
        
        .time {
            display: block;
            width: 300px;
            height: 50px;
            line-height: 50px;
            text-align: center;
            color: white;
            background: green;
            font-size: 30px;
        }
        
        .btn {
            width: 100%;
            margin-top: 30px;
            text-align: center;
        }
        
        button {
            color: white;
            font-size: 16px;
            border: 0;
        }
        
        button:nth-child(1) {
            background: green;
        }
        
        button:nth-child(2) {
            background: red;
        }
        
        button:hover {
            background: gray;
        }
    </style>
</head>

<body>
    <div id="demo">
        <div>
            <span class="time">00:00:00</span>
        </div>
        <div class="btn">
            <button onclick="start()">开始计时</button>
            <button onclick="stop()">停止计时</button>
        </div>
    </div>
    <script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
    <script>
        //将秒转换为时分秒
        function formatSeconds(value) {
            var hours = 0; // 时
            var minute = 0; // 分
            var second = parseInt(value); // 秒
            if (second > 60) {
                minute = parseInt(second / 60);
                second = parseInt(second % 60);
                if (minute > 60) {
                    hours = parseInt(minute / 60);
                    minute = parseInt(minute % 60);
                }
            }
            //如果值小于10，在前面补零
            if (hours < 10) {
                hours = "0" + hours;
            }
            if (minute < 10) {
                minute = "0" + minute
            }
            if (second < 10) {
                second = "0" + second;
            }
            return hours + ":" + minute + ":" + second;
        }
        //定时器
        var timer;
        //累加器
        var count;
        //开始计时
        function start() {
            console.log("开始计时");
            //清除定时器,防止多个定时器执行
            clearTimeout(timer);
            //初始化count值
            count = 0
            timer = setTimeout(function() {
                ++count;
                let result = formatSeconds(count);
                $(".time").html(result);
            }, 1000);
        }
        //停止计时
        function stop() {
            console.log("停止计时");
            //清除定时器
            clearTimeout(timer);
            $(".time").html(formatSeconds(0));
        }
    </script>

</body>

</html>