Current session timezone

-- Get
select sessiontimezone from dual;
--Db timezone
select dbtimezone from dual;
-- Set
set ora_sdtz=db_tz
or
alter session set time_zone=dbtimezone;
--change db time_zone
16:51:23 SQL> alter database set time_zone='+01:00';

Database altered.
16:55:11 SQL> create table t_lt (id timestamp with local time zone);

Table created.

Elapsed: 00:00:00.64
16:55:29 SQL> alter database set time_zone='+01:00';

Database altered.

Elapsed: 00:00:00.06
16:55:31 SQL> alter database set time_zone='+02:00';

Database altered.
16:56:16 SQL> insert into t_lt select systimestamp from dual;

1 row created.

Elapsed: 00:00:00.07
16:56:33 SQL> commit;

Commit complete.

Elapsed: 00:00:00.00
16:56:35 SQL> alter database set time_zone='+01:00';
alter database set time_zone='+01:00'
*
ERROR at line 1:
ORA-30079: cannot alter database timezone when database has TIMESTAMP WITH LOCAL TIME ZONE columns