document.getElementById('date_sel').onchange = function(){ var time = new Date(); var pre_year = time.getFullYear(); var pre_month = time.getMonth(); var pre_day = time.getDate(); var sel_day = parseInt(document.getElementById('date_sel').value); switch(sel_day){ case 0: pre_day +=1;break; case 1: pre_day +=2;break; case 2: pre_day +=3;break; case 3: pre_day +=4;break; default:pre_day +=1;break; } var endtime = new Date(pre_year, pre_month, pre_day); document.getElementById('lastdate').innerHTML = endtime.getFullYear()+"-"+(endtime.getMonth()+1)+"-"+endtime.getDate(); }; script>