<div id="myDiv"></div>
<script>
myDiv.innerHTML = /\d\d:\d\d:\d\d/.exec(new Date().toString())[0];
setInterval(function(){
myDiv.innerHTML = /\d\d:\d\d:\d\d/.exec(new Date().toString())[0]; 
},500);
</script>

<div id="myDiv">
<label for="set"><input type="number" id="set" step="1" value="0">秒</label>
<button id="btn">确定</button>
<button id="reset">重置</button> 
</div>
<script>
var timer;
reset.onclick = function(){
history.go();
}
btn.onclick = function(){
if(timer) return;
set.setAttribute('disabled','disabled');
timer = setInterval(function(){
if(Number(set.value) === 0){
clearInterval(timer);
timer = 0;
set.removeAttribute('disabled');
return;
}
set.value = Number(set.value) - 1;
},1000);
} 
</script>

<div id="myDiv">
<label for="hour"><input type="number" id="hour" min="0" max="23" step="1" value="0" />时</label>
<label for="minute"><input type="number" id="minute" min="0" max="59" step="1" value="0" />分</label>
<label for="second"><input type="number" id="second" min="0" max="23" step="1" value="0" />秒</label>
<button id="btn">确定</button>
<button id="reset">重置</button>
</div>
<script>
var timer;
//输入限制
hour.onchange = function(){
if(Number(this.value) !== Number(this.value)) this.value = 0;
if(this.value > 23) this.value = 23;
if(this.value < 0) this.value = 0;
}
second.onchange = minute.onchange = function(){
if(Number(this.value) !== Number(this.value)) this.value = 0;
if(this.value > 59) this.value = 59;
if(this.value < 0) this.value = 0;
}
reset.onclick = function(){
history.go();
}
btn.onclick = function(){
if(timer) return;
for(var i = 0; i < 3; i++){
myDiv.getElementsByTagName('input')[i].setAttribute('disabled','disabled');
}
//原始储存值
var setOri = hour.value*3600 + minute.value*60 + second.value*1;
//原始系统时间值
var timeOri = (new Date()).getTime();
//现在所剩时间值
var setNow;
cancelAnimationFrame(timer);
timer = requestAnimationFrame(function fn(){
//当前系统时间值
var timeNow = (new Date()).getTime();
//使系统时间的差值与设置时间的差值相等，来获得正常的时间变化
setNow = setOri - Math.floor((timeNow - timeOri)/1000);
hour.value = Math.floor((setNow%86400)/3600);
minute.value = Math.floor((setNow%3600)/60);
second.value = Math.floor(setNow%60);
timer = requestAnimationFrame(fn);
if(setNow==0){
cancelAnimationFrame(timer);
timer = 0;
btn.innerHTML = '计时结束';
for(var i = 0; i < 3; i++){
myDiv.getElementsByTagName('input')[i].removeAttribute('disabled');
}
setTimeout(function(){
btn.innerHTML = '确定';
},1000) 
}
})
}
</script>

<div id="myDiv">
<label for="set"><input id="set" value="0"></label>
<button id="btn">开始</button>
<button id="reset">重置</button> 
</div>
<script>
var timer;
var con = 'off';
var num = 0;
reset.onclick = function(){
history.go();
}
btn.onclick = function(){
if(con === 'off'){
set.setAttribute('disabled','disabled');
con = 'on';
btn.innerHTML = '暂停';
timer = setInterval(function(){
num+= 100;
var minute = Math.floor(num/1000/60);
var second = Math.floor(num/1000);
var ms = Math.floor(num00)/100;
set.value = minute + ' : ' + second + ' . ' + ms; 
},100);
}else{
clearInterval(timer);
con = 'off';
btn.innerHTML = '开始'; 
}
} 
</script>

<div id="myDiv">
<label for="set"><input id="set" value="0"></label>
<button id="btn">开始</button>
<button id="reset">重置</button> 
</div>
<script>
var timer;
var con = 'off';
//ori表示初始的系统时间
var ori;
//dis表示当前运行时的秒数(动态)
var dis = 0;
//last储存暂停时的秒数(静态)
var last = 0;
reset.onclick = function(){
history.go();
}
btn.onclick = function(){
if(con === 'off'){
set.setAttribute('disabled','disabled');
con = 'on';
btn.innerHTML = '暂停';
//保留已经走过的秒数的系统时间
ori = (new Date()).getTime() - last; 
timer = requestAnimationFrame(function fn(){
dis = (new Date()).getTime() - ori;
cancelAnimationFrame(timer);
timer = requestAnimationFrame(fn);
var minute = Math.floor(dis/1000/60);
var second = Math.floor(dis/1000);
var ms = Math.floor(dis00);
set.value = minute + ' : ' + second + ' . ' + ms; 
});
}else{
cancelAnimationFrame(timer);
btn.innerHTML = '开始'; 
con = 'off';
last = dis;
}
} 
</script>

<div id="myDiv"></div>
<div id="con">
<label for="hour"><input type="number" id="hour" min="0" max="23" step="1" value="0" />时</label>
<label for="minute"><input type="number" id="minute" min="0" max="59" step="1" value="0" />分</label>
<label for="second"><input type="number" id="second" min="0" max="23" step="1" value="0" />秒</label>
<button id="btn">确定</button>
<button id="reset">重置</button>
</div>
<div id="show"></div>
<script>
var timer;
//所剩时间
var dis;
myDiv.innerHTML = /\d\d:\d\d:\d\d/.exec(new Date().toString())[0];
setInterval(function(){
myDiv.innerHTML = /\d\d:\d\d:\d\d/.exec(new Date().toString())[0]; 
},100);
reset.onclick = function(){
history.go();
}
btn.onclick = function(){
//原始储存值
var setOri = hour.value*3600 + minute.value*60 + second.value*1;
//原始值转换为1970年的毫秒数
var setMs = +new Date(new Date().toDateString()) + setOri*1000;
//如果设置的时间早于当前时间，则设置无效
if(setMs < +new Date()){
return;
}
for(var i = 0; i < 3; i++){
con.getElementsByTagName('input')[i].setAttribute('disabled','disabled');
}
cancelAnimationFrame(timer);
timer = requestAnimationFrame(function fn(){
//算出设置时间与当前时间的差值
dis = Math.ceil((setMs - (new Date()).getTime())/1000);
var showHour = Math.floor((dis%86400)/3600);
var showMinute = Math.floor((dis%3600)/60);
var showSecond = Math.floor(dis%60);
timer = requestAnimationFrame(fn);
show.innerHTML = '距离预定时间还有 ' + showHour + '小时 ' + showMinute + '分 ' + showSecond + '秒';
//当差值为0时，时间到
if(dis==0){
cancelAnimationFrame(timer);
btn.innerHTML = '时间到了';
for(var i = 0; i < 3; i++){
con.getElementsByTagName('input')[i].removeAttribute('disabled');
}
timer = setTimeout(function(){
btn.innerHTML = '确定';
},1000) 
}
});
}
</script>