How does OCIDateTimeConstruct() create time values for OCI_DTYPE_DATE (DATE column)

Hello all,
Oracle version 11.1.0.6.0

Please help as I find very weird behavior and there is not enough example or document to explain:

This is my test table:

SQL> desc test_date;
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- -------------
 CVARCHAR2                                                      VARCHAR2(10)
 CDATE                                                 NOT NULL DATE
 
I called this to initialize my date value:
OCIDateTimeConstruct(envhp, errhp, v_datetime, 2012, 7, 18, 0, 0, 0, 0, NULL, 0);

After insert using OCIBindByName(), my table has:

SQL> select cvarchar2, cdate, dump(cdate, 10) from test_date;
CVARCHAR2  CDATE
---------- ---------
DUMP(CDATE,10)
-----------------------------------------------------------------------------
AAA        18-JUL-12
Typ=12 Len=7: 120,112,7,18,0,0,0

[Query 1]
SQL> select * from test_date where cdate = '18-JUL-12';
no rows selected

[Query 2]
SQL> select * from test_date where cdate < '18-JUL-12';