prova qualcosa del genere:
codice:
<html> 
<head> 
<title>...</title> 
<script>
function s(x){
if(!/^[0-9]{2}\/[0-9]{4}$/.test(x))alert("Formato data errato")
else{
	x = x.split("/");
	if(x[1]>2003)if(x[0]>8)alert("Data troppo futura!!");
	}
}
</script>
</head> 
<body> 
<input type="text" onBlur="s(this.value)">
</body>
</html>