aqui te dejo un ejemplo, con el codigo que pusiste
Código PHP:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
<script language="javascript">
function comparar(){
with(document.forms[0]){
var auxfec1=Date.parse(an.value+'/'+me.value+'/'+di.value);
var auxfec2=Date.parse(ano.value+'/'+mes.value+'/'+dia.value);
}
if (auxfec1<auxfec2){
alert('fecha2 mayor que fecha1');
}
else if(auxfec1>auxfec2){
alert('fecha1 mayor que fecha2');
}
else{
alert('fecha1 es igual a fecha2');
}
}
</script>
</head>
<body>
<div align="center">
<table width="630" border="0" class="contact">
<tr>
<td width="620"><form action="esp_its3.php" method="post">
<table width="608" border="0" align="center" cellpadding="0" cellspacing="3">
<tr>
<td colspan="3"><div align="left"></div>
</td>
</tr>
<tr>
<td width="134"> </td>
<td width="187"> </td>
<td width="243"><div align="center" class="its">
<div align="center">d/m/a</div>
</div></td>
</tr>
<tr>
<td valign="top" class="formu_txt_1"> </td>
<td valign="top" class="formu_txt_1"><p>Llegada</p></td>
<td><div align="left"> <?php
$an=date("Ye");
echo "Año:";
echo "<select name='an'>";
for($i=2008; $i<=2011; $i++){
echo "<option value='".$i."' ";
if($an==$i){
echo "selected";
}
echo ">".$i."</option>";
}
echo "</select>";
$me=date("ms");
echo "Mes:";
echo "<select name='me'>";
for($i=1; $i<=12; $i++){
echo "<option value='".$i."' ";
if($me==$i){
echo "selected";
}
echo ">".$i."</option>";
}
echo "</select>";
echo "Dia:";
echo "<select name='di'>";
$di=date("da");
for($i=1; $i<=31; $i++){
echo "<option value='".$i."' ";
if($di==$i){
echo "selected";
}
echo ">".$i."</option>";
}
echo "</select>";
?>
</div></td>
</tr>
<tr>
<td valign="top" class="formu_txt_1"> </td>
<td valign="top" class="formu_txt_1">Salida</td>
<td><div align="left">
<?php
$ano=date("Y");
echo "Año:";
echo "<select name='ano'>";
for($i=2008; $i<=2011; $i++){
echo "<option value='".$i."' ";
if($ano==$i){
echo "selected";
}
echo ">".$i."</option>";
}
echo "</select>";
$mes=date("m");
echo "Mes:";
echo "<select name='mes'>";
for($i=1; $i<=12; $i++){
echo "<option value='".$i."' ";
if($mes==$i){
echo "selected";
}
echo ">".$i."</option>";
}
echo "</select>";
echo "Dia:";
echo "<select name='dia'>";
$dia=date("d");
for($i=1; $i<=31; $i++){
echo "<option value='".$i."' ";
if($dia==$i){
echo "selected";
}
echo ">".$i."</option>";
}
echo "</select>";
?>
</div></td>
</tr>
<tr>
<td colspan="3"><hr size='1' color='#D9ECFF' /></td>
</tr>
</table>
<table width="608" border="0">
<tr>
<td><hr size='1' color='#D9ECFF' /></td>
</tr>
<tr>
<td><div align="center"><span class="Estilo8">
<input name="enviar2" type="button" id="enviar2" value="Siguiente" onclick="comparar()" />
<input name="borrar2" type="reset" id="borrar2" value="Borrar" />
</span></div></td>
</tr>
</table>
</form></td>
</tr>
</table>
<p> </p>
</div>
</body>
</html>