Ho fatto così:
<html>
<head>
<meta http-equiv="refresh" content="20"; URL="http://michiross.altervista.org/redirect.php">
</head>
<body>
<?php
$time=gmdate("H:i");
print("$time");
if($time=="21:18"){
echo "<script type='text/javascript'>"."window.open('www.mysite.com');</script>";
?>
</body>
</html>

Ma ora mi da:
Parse error: syntax error, unexpected $end in /membri/michiross/redirect.php on line 13
(Non so neanche io perchè le ho separate.)