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>