JavaScript: Formatar campo data

<script type="text/javascript">
function mascara_data(campo, valor){
  var mydata = '';
  mydata = mydata + valor;
  if (mydata.length == 2){
    mydata = mydata + '/';
    campo.value = mydata;
  }
  if (mydata.length == 5){
    mydata = mydata + '/';
    campo.value = mydata;
  }
}
</script>

<script type="text/javascript"> function mascara_data(campo, valor){ var mydata = ''; mydata = mydata + valor; if (mydata.length == 2){ mydata = mydata + '/'; campo.value = mydata; } if (mydata.length == 5){ mydata = mydata + '/'; campo.value = mydata; } } </script>

<form name="form" action="#" method="post">
Data 1 <input type="text" name="data" onkeyup="mascara_data(this, this.value)" maxlength="10" />
<br />
Data 2 <input type="text" name="data1" onkeyup="mascara_data(this, this.value)" maxlength="10" />
<br />
Data 3 <input type="text" name="data2" onkeyup="mascara_data(this, this.value)" maxlength="10" />
<br />
<input type="submit" value="ENVIAR" />
</form>
<?
if ($_POST){
  echo $_POST['data'];
}
?>

<form name="form" action="#" method="post"> Data 1 <input type="text" name="data" onkeyup="mascara_data(this, this.value)" maxlength="10" /> <br /> Data 2 <input type="text" name="data1" onkeyup="mascara_data(this, this.value)" maxlength="10" /> <br /> Data 3 <input type="text" name="data2" onkeyup="mascara_data(this, this.value)" maxlength="10" /> <br /> <input type="submit" value="ENVIAR" /> </form> <? if ($_POST){ echo $_POST['data']; } ?>