Question

num_daysopen returns how many days since dateopened.
Days left returns how many days a user has to close something (they have upto 28 days from dateopened). If its longer than 28 days days_left needs to default to 0 not -5 or -15 etc.

SELECT
  DATEDIFF(DATE(NOW()), DATE(dateopened)) AS num_daysopen,
  (28 - DATEDIFF(DATE(NOW()), DATE(dateopened))) as days_left
FROM table

Answer 1

Use GREATEST().

SELECT
  DATEDIFF(DATE(NOW()), DATE(dateopened)) AS num_daysopen,
  GREATEST(0,(28 - DATEDIFF(DATE(NOW()), DATE(dateopened)))) as days_left
FROM table