Calendar plug-in developed by native js

<style type="text/css">
  * {
   margin: 0;
   padding: 0;
  }
  ul, li {
   list-style: none;
  }
  #week {
   width: 350px;
   height: 350px;
   border: 1px solid #ccc;
  }
  #week h6 {
   font-size: 20px;
   overflow: hidden;
   height: 40px;
   line-height: 40px;
  }
  #week h6 div {
   float: left;
   width: 250px;
   text-align: center;
  }
  #week .prev {
   float: left;
   font-size: 12px;
   width: 50px;
   cursor: pointer;
  }
  #week .next {
   float: right;
   font-size: 12px;
   width: 50px;
   cursor: pointer;
  }
  #week p span {
   width: 50px;
   float: left;
   text-align: center;
   height: 30px;
   line-height: 30px;
  }
  #week p {
   background: #ccc;
   overflow: hidden;
   height: 30px;
  }
  #week ul {
   margin-top: 10px;
  }
  #week ul li {
   width: 50px;
   float: left;
   text-align: center;
   height: 30px;
   cursor: pointer;
  }
 </style>
<input type="text" id="ele" onfocus="currentWeek(this)"/>
<script>
 function currentWeek(ele) {
  var obj = offset(ele);
  var x = obj.left;
  var y = obj.top + ele.offsetHeight + 1;
  //创建日历界面
  if (!document.getElementById(&#39;week&#39;)) {
   var oDiv = document.createElement(&#39;div&#39;);
   document.body.appendChild(oDiv);
   oDiv.id = &#39;week&#39;;
   oDiv.style.top = y + "px";
   oDiv.style.left = x + "px";
   //创建日历title
   var h6 = document.createElement(&#39;h6&#39;);
   oDiv.appendChild(h6);
   var prev = document.createElement(&#39;div&#39;);
   h6.appendChild(prev);
   prev.className = &#39;prev&#39;;
   prev.innerHTML = &#39;上个月&#39;;
   var content = document.createElement(&#39;div&#39;);
   content.className = "content";
   h6.appendChild(content);
   var next = document.createElement(&#39;div&#39;);
   h6.appendChild(next);
   next.className = &#39;next&#39;;
   next.innerHTML = &#39;下个月&#39;;
   //创建星期日到星期六的文字行
   var oPs = document.createElement(&#39;p&#39;);
   oPs.className = "rlTitle";
   oDiv.appendChild(oPs);
   var opsCont = [&#39;日&#39;, &#39;一&#39;, &#39;二&#39;, &#39;三&#39;, &#39;四&#39;, &#39;五&#39;, &#39;六&#39;];
   for (var i = 0; i <= 6; i++) {
    var oSpan = document.createElement(&#39;span&#39;);
    oPs.appendChild(oSpan);
    oSpan.innerHTML = opsCont[i];
   }
   //创建日历上面的日期行数所需要的变量
   var oUl = document.createElement(&#39;ul&#39;);
   oUl.className = "rlCenter";
   oDiv.appendChild(oUl);
   var currentDate = new Date();
   var currentYear = currentDate.getFullYear();
   var currentMonth = currentDate.getMonth();
   active(currentMonth);//传参数月份
   //创建日历上下翻月
   prev.onclick = function () {
    active(--currentMonth);
   };
   next.onclick = function () {
    active(++currentMonth);
   };
  }
  //动态创建日历上面日期，变包装成方法
  function active(m) {
   oUl.innerHTML = &#39;&#39;; //切忌一定要把这个内容去掉，要不然会点一次翻页都在日历下面依次显示出来
   var activeDate = new Date(currentYear, m, 1); //外面传进来的不断变化的日期对象
   var year = activeDate.getFullYear();
   var month = activeDate.getMonth(); //把当前的月份保存下来只是为了给title获取月份
   content.innerHTML = year + &#39;年&#39; + (month + 1) + &#39;月&#39;;
   //创建日历上面的日期行数
   var n = 1 - activeDate.getDay();
   if (n == 1) {
    n = -6;
   } //为了日历更友好的显示三个月，让用户看的更明白。
   activeDate.setDate(n); //如果n为负数，则减少月份.在用这个月最后一天减去这个值就可以获得日历从哪天开始的。
   for (var i = 0; i < 42; i++) {
    var oLi = document.createElement(&#39;li&#39;);
    oUl.appendChild(oLi);
    var date = activeDate.getDate(); //返回日期1-31号
    oLi.innerHTML = date;
    oLi.dateValue = year + "-" + (activeDate.getMonth() + 1) + "-" + date; //这里必须是activeDate.getMonth()+1，不能用m+1。因为这个是一直变化的。要不然日历不管点哪天都是属于当前月份的。
    oLi.onclick = function () {
     ele.value = this.dateValue;//文本框获取的年月日
     document.body.removeChild(oDiv); //获取到年月日，日历取消
     oDiv = null;
    };
    if (activeDate.getMonth() != month) {
     oLi.style.color = "#ccc"; //不是本月的天数颜色变成灰色  
    }
    //切忌下面这个增加天数语句，一定要判断完上面是不是本月的天数，然后在添加这条增加语句，要不然会出现错误。
    activeDate.setDate(date + 1); //如果超出该月份应有的天数则增加月份
   }
  }
 }
 function offset(ele) {
  var l = ele.offsetLeft;
  var t = ele.offsetTop;
  var p = ele.offsetParent;
  while (p) {
   if (window.navigator.userAgent.indexOf("MSIE 8") > -1) {
    l += p.offsetLeft;
    t += p.offsetTop;
   } else {
    l += p.offsetLeft + p.clientLeft;
    t += p.offsetTop + p.clientTop;
   }
   p = p.offsetParent;
  }
  return {
   left: l,
   top: t
  }
 }
</script>