you don't need to have a hidden form element for date, just pop it in the query
$sql = mysql_query("INSERT INTO members (username, gender, birthday, email, password, ipaddress, sign_up_date)
VALUES('$username','$gender','$bday','$email','$db_password', '$ipaddress', now())")
I have a field named sign_up_date in mysql, and the type is date NOT NULL default '0000-00-00'
notice in the VALUES section I have now()...thats all thats needed for php to add in the current date when your query is run