I recently did a little project to display a coupon with an expiration date that was calculated. It just adds 21 days to today's date and prints it on the coupon. Here is what the code looks like:
Coupon expires ' ; echo date('Y-m-d', $nextWeek) ; echo '
Because it adds 21 days to todays date, your output date will be different. I just put this in a story and set the input format to php code.