获取当前系统时间，自动更新

将日期转换为指定的格式：比如转换成 年月日时分秒 这种格式

setInterval(function(){

    var d = new Date()  
        var year = d.getFullYear()
        var month = d.getMonth() + 1
        var day = d.getDate()
        var hour = d.getHours()
        var min = a(d.getMinutes())
        var sec = a(d.getSeconds())
        var week = d.getDay()
    
        var arr = ["星期日","星期一","星期二","星期三","星期四","星期五","星期六"]
        
        function a(m){
            if(m < 10){
            return "0" + m
        }else return m
     }
         //当分和秒小于0时，自动补0
         
        box.innerHTML =   year + "年" + month + "月" + day + "日" + " " + hour + ":" + min + ":" + sec + " " + arr[week] 
     
    },1000)