手写JS 点击获取某一天的时间

getDay(-7);  // 20200923

getDay(0);  //  20200930

 function getDay(day) {  
   let today = new Date();  
   let targetday_milliseconds=today.getTime() + 1000*60*60*24*day;       
   today.setTime(targetday_milliseconds);
   var tYear = today.getFullYear();  
   var tMonth = today.getMonth();  
   var tDate = today.getDate();  
   tMonth = this.doHandleMonth(tMonth + 1);  
   tDate = this.doHandleMonth(tDate);  
   return String(tYear)+String(tMonth)+String(tDate);  
},
function doHandleMonth(month){  
  let m = month;  
  if(month.toString().length == 1){  
      m = '0' + month;  
  }  
  return m;  
},