//指定时间减2秒
function reduceTwoS(dateStr){//dateStr格式为yyyy-mm-dd hh:mm:ss
   var dt=new Date(dateStr.replace(/-/,"/"));//将传入的日期格式的字符串转换为date对象 兼容ie
  //    var dt=new Date(dateStr);//将传入的日期格式的字符串转换为date对象 非ie
   var ndt=new Date(dt.getTime()-2000);//将转换之后的时间减去两秒
   var result={
     year:parseInt(ndt.getFullYear()),
     month:parseInt(ndt.getMonth()+1),
     day:parseInt(ndt.getDay()),
     hour:parseInt(ndt.getHours()),
     minute:parseInt(ndt.getMinutes()),
     second:parseInt(ndt.getSeconds())
   }
  return result;
}

#求时间差改了，改为求半小时前
function showtime(){
var dateStr=‘2017-07-18 13:21:06‘;
var dt=new Date(dateStr.replace(/-/,"/"));
var date=new Date(dt.getTime()-(30*60*1000));
var seperator1 = "-";
    var seperator2 = ":";
    var month = date.getMonth() + 1;
    var strDate = date.getDate();
    if (month >= 1 && month <= 9) {
        month = "0" + month;
    }
    if (strDate >= 0 && strDate <= 9) {
        strDate = "0" + strDate;
    }
    var currentdate = date.getFullYear() + seperator1 + month + seperator1 + strDate
            + " " + date.getHours() + seperator2 + date.getMinutes()
            + seperator2 + date.getSeconds();
    return currentdate;
}