How to Subtract Days in MySQL

SELECT DATE(NOW()-INTERVAL 15 DAY)

For a list of units see http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html#function_date-add

Not entirely related to this question but is related to the title:

SELECT SUBTIME("10:24:21", "5");        -- subtracts 5 seconds. (returns "10:24:16")

SELECT SUBTIME("10:24:21", "01:00:00");  -- subtracts one hour. (returns "09:24:21")

Documentation: MySQL SUBTIME function