The SQL DATE_SUB is a mySql function, unlike SQL DATE_ADD function which add time value, SQL DATE_SUB will subtract time values (intervals) from a date value.
SQL DATE_SUB Syntax
DATE_SUB(date-expression, INTERVAL expr type)
For the type, please refer to the SQL ADDDATE() function.
SQL DATE_SUB Examples
SELECT DATE_SUB('2010-08-12', INTERVAL 3 DAY) AS NewDate
Result: 2010-08-09
SELECT DATE_SUB('2010-08-12', INTERVAL '3-2' YEAR_MONTH) AS NewDate
Result: 2007-06-12
SELECT DATE_SUB('2011-09-14 2:44:36', INTERVAL '2:26' HOUR_MINUTE) AS NewDate
Result: 2011-09-14 00:18:36