Date 日期格式化

 <span id="time"></span>
  <script>

 //名称：日期加法函数
 //参数：part(year、month、day、hour、minute、second、millisecond)
 //返回：Date对象
 Date.prototype.add = function (part, value) {
     if (!value || isNaN(value)) value = 0;
     switch (part) {
         case "year":
             this.setFullYear(this.getFullYear() + value);
             break;
         case "month":
             this.setMonth(this.getMonth() + value);
             break;
         case "day":
             this.setDate(this.getDate() + value);
             break;
         case "hour":
             this.setHours(this.getHours() + value);
             break;
         case "minute":
             this.setMinutes(this.getMinutes() + value);
             break;
         case "second":
             this.setSeconds(this.getSeconds() + value);
             break;
         case "millisecond":
             this.setMilliseconds(this.getMilliseconds() + value);
             break;
         default:
     }
     return this;
 };

 Date.prototype.addYears = function (value) {
     if (!value || isNaN(value)) value = 0;
     this.setFullYear(this.getFullYear() + value);
     return this;
 };

 Date.prototype.addMonths = function (value) {
     if (!value || isNaN(value)) value = 0;
     this.setMonth(this.getMonth() + value);
     return this;
 };

 Date.prototype.addDays = function (value) {
     if (!value || isNaN(value)) value = 0;
     this.setDate(this.getDate() + value);
     return this;
 };

 Date.prototype.addHours = function (value) {
     if (!value || isNaN(value)) value = 0;
     this.setHours(this.getHours() + value);
     return this;
 };

 Date.prototype.addMinutes = function (value) {
     if (!value || isNaN(value)) value = 0;
     this.setMinutes(this.getMinutes() + value);
     return this;
 };

 Date.prototype.addSeconds = function (value) {
     if (!value || isNaN(value)) value = 0;
     this.setSeconds(this.getSeconds() + value);
     return this;
 };

 Date.prototype.addMilliseconds = function (value) {
     if (!value || isNaN(value)) value = 0;
     this.setMilliseconds(this.getMilliseconds() + value);
     return this;
 };

 //名称：日期加法函数
 //参数：time(日期字符串，示例：12:00:00)
 //返回：Date对象
 Date.prototype.addTime = function (time) {
     var timeRegex = /^([0-1]?\d|2[0-3])(:[0-5]?\d){1,2}$/g;
     if (timeRegex.test(time)) {
         var value = Date.parse("1970/1/1 " + time) - Date.parse("1970/1/1");
         this.setMilliseconds(this.getMilliseconds() + value);
     }
     return this;
 };

 //名称：日期格式化函数
 //参数：format(示例：yyyy-MM-dd hh:mm:ss)、zeroize(是否补零)
 //返回：日期字符串
 Date.prototype.toCustomString = function (format, zeroize) {
     if (!zeroize) zeroize = false;
     var dy = this.getFullYear();
     var dM = this.getMonth() + 1;
     var dd = this.getDate();
     var dh = this.getHours();
     var dm = this.getMinutes();
     var ds = this.getSeconds();
     var dS = this.getMilliseconds();
     var orm = {
         "y+": dy.toString(),
         "M+": !zeroize ? dM.toString() : dM < 10 ? '0' + dM : dM.toString(),
         "d+": !zeroize ? dd.toString() : dd < 10 ? '0' + dd : dd.toString(),
         "h+": !zeroize ? dh.toString() : dh < 10 ? '0' + dh : dh.toString(),
         "m+": !zeroize ? dm.toString() : dm < 10 ? '0' + dm : dm.toString(),
         "s+": !zeroize ? ds.toString() : ds < 10 ? '0' + ds : ds.toString(),
         "S": dS.toString()
     };
     for (var i in orm) {
         var patt = new RegExp(i);
         if (patt.test(format)) {
             var item = orm[i];
             var ms = format.match(patt);
             var result = ms[0];
             if (i === "S") {
                 format = format.replace(result, item);
             } else {
                 format = format.replace(result, item.substr(item.length - result.length));
             }
         }
     }
     return format;
 };
 window.onload = function(){
     var time = document.getElementById("time");

     setInterval('time.innerText = new Date().toCustomString("yyyy-MM-dd hh:mm:ss")',1000);

 }

 </script>

没有格式啥的要求的话，就用Date下的toLocaleString()显示年月日时间或者toLocaleDateString()显示年月日

 <span id="time"></span>
  <script>
  var time = document.getElementById("time");
  setInterval('time.innerHTML = new Date().toLocaleString()', 1000);
 </script>