SQL> SET ECHO ON;
SQL> ALTER SESSION SET nls_date_format = 'DD.MM.YYYY HH24:MI:SS';

Session altered.

SQL> 
SQL> CREATE OR REPLACE VIEW my_halfhour_trunc_good
  2  AS
  3  with t as (
  4  select to_date('02.09.2015 19:29:59','dd.mm.yyyy hh24:mi:ss') d from dual 
  5  union all
  6  select to_date('02.09.2015 19:49:59','dd.mm.yyyy hh24:mi:ss') d from dual
  7  )
  8  select trunc(d,'hh24') + case when to_char(d,'mi')>'30' then 1/24/2 else 0 end dd from t
  9  ;
View MY_HALFHOUR_TRUNC_GOOD created.

SQL> DESCRIBE my_halfhour_trunc_good;

Name Null? Type 
---- ----- ---- 
DD         DATE 

SQL> SELECT * FROM my_halfhour_trunc_good;

DD                 
-------------------
02.09.2015 19:00:00
02.09.2015 19:30:00

SQL> 
SQL> CREATE OR REPLACE VIEW my_halfhour_trunc_bad
  2  AS
  3  with t as (
  4  select to_date('02.09.2015 19:29:59','dd.mm.yyyy hh24:mi:ss') d from dual 
  5  union all
  6  select to_date('02.09.2015 19:49:59','dd.mm.yyyy hh24:mi:ss') d from dual
  7  )
  8  select to_char(d,'dd.mm.yyyy hh24')||':'||Trunc(To_char(d,'mi')/10)||'0' AS dd from t;
View MY_HALFHOUR_TRUNC_BAD created.

SQL> DESCRIBE my_halfhour_trunc_bad;

Name Null? Type         
---- ----- ------------ 
DD         VARCHAR2(55) 

SQL> SELECT * FROM my_halfhour_trunc_bad;

DD                                                     
-------------------------------------------------------
02.09.2015 19:20
02.09.2015 19:40