How to Hack Core Files Properly

Hi

I need to change the time-out cookie for protected post login from the default of 10 days to one hour.

The code is in the main part of wp-login.php…

setcookie( 'wp-postpass_' . COOKIEHASH, $wp_hasher->HashPassword( stripslashes( $_POST['post_password'] ) ), time() + 3600, COOKIEPATH );

As you can see, I’ve changed it already but by editing the core file.

How do I implement this change properly?

Thanks

Mike V.