codice:
<input name="giorno" maxlength="2" onkeyup="if(this.value.length==2){document.getElementById('mm').focus();}" />

<input name="mese" maxlength="2" id="mm" onkeyup="if(this.value.length==2){document.getElementById('aaaa').focus();}" />

<input name="anno" maxlength="4" id="aaaa" />