[Deff]

<script type="text/javascript">
function f_Date(el,n) {
  el.value = el.value.replace(/[^0-9]/g,'');
  if(el.value.length>n)el.value=el.value.substr(n);
  if(el.value.length==n) el.nextSibling.nextSibling.focus();
}
</script>
<span id="date">
<input name="YYYY" oninput="f_Date(this,4)" placeholder="Введите год            2015"> 
<input name="MM"oninput="f_Date(this,2)" placeholder="Введите месяц      01-12"> 
<input name="DD" oninput="f_Date(this,2)" placeholder="Введите день          1-31"> 
<input type="submit" value="Отправить"> 
</span>