you would need to set the login page to store the arrival date and the departure date in session variables
in the access rules, you will want to use time() to return the current time stamp, and strtotime to convert the dates in the sesson variables to a time stamp
the access rule settings will look like:
Allow IF
Value: <?php echo strtotime($_SESSION['arrivalDate']); ?>
Criteria: >=
Compare To: <?php echo time(); ?>
Allow IF
Value: <?php echo strtotime($_SESSION['departureDate']); ?>
Criteria: <=
Compare To: <?php echo time(); ?>