SOURCE

function diffTime(startDate,endDate) {  
    var diff=endDate.getTime() - startDate.getTime();//时间差的毫秒数  
  
    //计算出相差天数  
    var days=Math.floor(diff/(24*3600*1000));  
       
    //计算出小时数  
    var leave1=diff%(24*3600*1000);    //计算天数后剩余的毫秒数  
    var hours=Math.floor(leave1/(3600*1000));  
    //计算相差分钟数  
    var leave2=leave1%(3600*1000);        //计算小时数后剩余的毫秒数  
    var minutes=Math.floor(leave2/(60*1000));  
       
    //计算相差秒数  
    var leave3=leave2%(60*1000);      //计算分钟数后剩余的毫秒数  
    var seconds=Math.round(leave3/1000);  
      
    var returnStr = seconds + "秒";  
    if(minutes>0) {  
        returnStr = minutes + "分" + returnStr;  
    }  
    if(hours>0) {  
        returnStr = hours + "小时" + returnStr;  
    }  
    if(days>0) {  
        returnStr = days + "天" + returnStr;  
    }  
    return returnStr;  
}
alert(diffTime(new Date('2019/9/19 09:30'), new Date('2019/9/19 12:30:47')))