If the second or the third argument of a BETWEEN predicate was
a constant expression, like '2005.09.01' - INTERVAL 6 MONTH,
while the other two arguments were fields then the predicate 
was evaluated incorrectly and the query returned a wrong
result set.
The bug was introduced in 5.0.17 when in the fix for 12612.


mysql-test/r/func_time.result:
  Added a test case for bug #18618.
mysql-test/t/func_time.test:
  Added a test case for bug #18618.