原生js计时器(时间格式计时器)

let hour,minute,second;//时 分 秒
hour=minute=second=0;//初始化
let millisecond=0;//毫秒
let int;
function Reset(){//重置
    window.clearInterval(int);
    millisecond=hour=minute=second=0;
    document.getElementById('timetext').value='00时00分00秒000毫秒';
}
function start(){//开始
    int=setInterval(timer,50);
}
function timer(){//计时
    millisecond=millisecond+50;
    if(millisecond>=1000){
        millisecond=0;
        second=second+1;
    }
    if(second>=60){
        second=0;
        minute=minute+1;
    }
    if(minute>=60){
        minute=0;
        hour=hour+1;
    }
    // document.getElementById('timetext').value=hour+':'+minute+':'+second+':'+millisecond+'毫秒';
    document.getElementById('timetext').value=hour+':'+minute+':'+second;
}
function stop(){//暂停
    window.clearInterval(int);
}

<div style="text-align: center">
<input type="text" id="timetext" value="00时00分00秒" readonly><br>
<button type="button" onclick="start()">开始</button> <button type="button" onclick="stop()">暂停</button> <button type="button" onclick="Reset()">重置</button>
</div>