Do I understand right that you assume that DATE_ADD("2011-01-01", INTERVAL 6 MONTH)
should give you '2011-06-30' instead of '2011-07-01'? Of course, 2011-01-01 + 6 months is 2011-07-01. You want something like DATE_SUB(DATE_ADD("2011-01-01", INTERVAL 6 MONTH), INTERVAL 1 DAY)
.