The MySQL DATE_ADD syntax is:
DATE_ADD(date, INTERVAL, expr interval)
Where 'date' is a date string or temporal data type representation (in MySQL TIMESTAMP, DATE or DATETIME format, or a similar string which can be used as such), 'expr' is an integer and 'interval' is one of: SECOND, MINUTE, HOUR, DAY, MONTH, YEAR, MINUTE_SECOND, HOUR_MINUTE, HOUR_SECOND, DAY_HOUR, DAY_MINUTE, DAY_SECOND or YEAR_SECOND.
Suppose your table column is named 'checkout' (a temporal column type of DATE) and you want to display a date 90 days from 'checkout' as 'due date':
SELECT DATE_ADD(checkout, INTERVAL 90 DAY) AS 'Due Date' FROM some_table WHERE ... yada-yada-yada...