Daylight saving error - oracle

Question

I would like to calculate time diff between to dates in a different time zone. I am converting this two times to Greenwich time. But when I am converting dates in Europe/Moscow timezone the offset is different although there is no daylight saving.
This is the case I checked:
SELECT TO_DATE('5/20/2018 10:05:00 PM','mm/dd/yyyy hh:mi:ss am'),
TO_DATE('5/20/2018 10:05:00 PM','mm/dd/yyyy hh:mi:ss am')
- extract(TIMEZONE_HOUR from from_tz (TO_TIMESTAMP('5/20/2018 10:05:00 PM','mm/dd/yyyy hh:mi:ss am') ,'GMT' ) at time zone 'Europe/Moscow')/24 deptime_GRINICH
from dual
UNION
SELECT TO_DATE('5/24/2018 11:35:00 PM','mm/dd/yyyy hh:mi:ss am'),
TO_DATE('5/24/2018 11:35:00 PM','mm/dd/yyyy hh:mi:ss am')
- extract(TIMEZONE_HOUR from from_tz (TO_TIMESTAMP('5/24/2018 11:35:00 PM','mm/dd/yyyy hh:mi:ss am') ,'GMT' ) at time zone 'Europe/Moscow')/24 deptime_GRINICH
from dual
And the results are:
Date date converting to Greenwich time zone
5/20/2018 10:05:00 PM 5/20/2018 6:05:00 PM --> hour difference 4 hours
5/24/2018 7:35:00 PM 5/24/2018 3:35:00 PM --> hour difference 4 hours
but 2018 Time Zones - Sochi is UTC + 3h

Answer 1

In October 2014 Russia changed their rule for Daylight-Saving. Your Timezone file at Oracle Database could be an old one which does not cover the recent changes. Verify version with
SELECT * FROM V$TIMEZONE_FILE;
and consider an upgrade, see Upgrading the Time Zone File and Timestamp with Time Zone Data
However, it seems to be a bug in Oracle. Have a look at my query which is a bit more clear than your example:
SELECT *
FROM NLS_SESSION_PARAMETERS
WHERE parameter LIKE 'NLS_TIMESTAMP_TZ_FORMAT';
PARAMETER VALUE
-------------------------- --------------------------------------
NLS_TIMESTAMP_TZ_FORMAT YYYY-MM-DD HH24:MI:SSfmXFF3 fmTZH:TZM
1 row selected.
SELECT
EXTRACT(TIMEZONE_HOUR FROM TIMESTAMP '2018-05-20 22:05:00 Europe/Moscow') AS TZ_HOUR,
TO_CHAR(TIMESTAMP '2018-05-20 22:05:00 Europe/Moscow', 'YYYY-MM-DD HH24:MI:SS TZH:TZM') AS ts2,
TIMESTAMP '2018-05-20 22:05:00 Europe/Moscow' AS ts3
FROM dual;
TZ_HOUR TS2 TS3
---------- ------------------------------ ----------------------------------
4 2018-05-20 22:05:00 +04:00 20.05.2018 22:05:00.000000000 +03:00
1 row selected.
That's really strange, because TZH:TZM, resp. EXTRACT(TIMEZONE_HOUR FROM ...) returns different value than default NLS_TIMESTAMP_TZ_FORMAT = ... TZH:TZM. For other time zones (e.g. Europe/Zurich) I get always 02:00 - as expected.
You may open a ticket at Oracle support.
I have two databases, an old and a new one. The old one does not reflect recent changes in Russian Daylight-Saving times, the newer one does. However, the error as above appears on both:
ALTER SESSION SET NLS_TIMESTAMP_TZ_FORMAT = 'YYYY-MM-DD HH24:MI:SS TZH:TZM';
SELECT filename, VERSION,
TO_CHAR(TIMESTAMP '2018-05-20 22:05:00 Europe/Moscow', 'TZH:TZM TZD') AS ts1,
TIMESTAMP '2018-05-20 22:05:00 Europe/Moscow' AS ts2,
TO_CHAR(TIMESTAMP '2018-01-20 22:05:00 Europe/Moscow', 'TZH:TZM TZD') AS ts3,
TIMESTAMP '2018-01-20 22:05:00 Europe/Moscow' AS ts4
FROM V$TIMEZONE_FILE;
Old (Daylight-Saving times still existing due to old timezlrg_14.dat file):
FILENAME VERSION TS1 TS2 TS3 TS4
---------------- -------- ----------- ------------------------------------ ----------- -------------------------------------
timezlrg_14.dat 14 +04:00 MSD 20.05.2018 22:05:00.000000000 +03:00 +03:00 MSK 20.01.2018 22:05:00.000000000 +03:00
New (no Daylight-Saving time changes):
FILENAME VERSION TS1 TS2 TS3 TS4
---------------- -------- ----------- ------------------------------------ ----------- -------------------------------------
timezlrg_18.dat 18 +04:00 MSK 20.05.2018 22:05:00.000000000 +03:00 +04:00 MSK 20.01.2018 22:05:00.000000000 +03:00