In the trigger for the discount, add 2 triggers:
If today is after a certain day:
2/28/2011
AND
If today is before a certain day:
4/1/2011
to show an error if the entered coupon code does not match the one in your discount rule, add another set session value behavior on the cart page, set the trigger to Before page load, set the name to something like realCode and set the value to the same value used in the discount rule.
then add an if statement to compare the two:
php:
<?php if(isset($_SESSION['PromoCode']) && isset($_SESSION['realCode']) && ($_SESSION['PromoCode'] != $_SESSION['realCode'])) { ?>
the promo code entered did not match
<?php } ?>
