function calc()
{
   dt=new Date();
   dy=dt.getYear();
   dm=dt.getMonth();
   dd=dt.getDate();
   if(dy<2000)dy+=1900;

   my=parseInt(document.tds_form.year.value);
   mm=document.tds_form.month.selectedIndex;
   da=document.tds_form.day.selectedIndex;
   d1=da*1;
   if((dy<my)||((dy==my)&&(dm<mm))||((dy==my)&&(dm==mm)&&(dd<d1))){
      alert("まだ生まれてません！");
      return;
      }
   if((d1==0)||(d1>31)){
      alert("日付が間違っています！");
      return;
      }
   old=dy-my;
   if(dm<mm)old--;
   else{
   if((dm=mm)&&(dd<d1))old--;
   }
   document.tds_form.age.value = old;
}