Oracle 11g Release 2

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CREATE TABLE your_table ( utcstamp ) AS
SELECT 1576666800 FROM DUAL
1 rows affected

ALTER SESSION SET NLS_TIMESTAMP_TZ_FORMAT = 'YYYY-MM-DD HH24:MI:SS.FF9 TZR';
ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
SELECT ( TIMESTAMP '1970-01-01 00:00:00 UTC' + utcstamp * INTERVAL '1' SECOND )
         AT TIME ZONE 'Europe/Amsterdam' AS Amsterdam_Time
FROM   your_table


AMSTERDAM_TIME


2019-12-18 12:00:00.000000000 EUROPE/AMSTERDAM


SELECT CAST(
         ( TIMESTAMP '1970-01-01 00:00:00 UTC' + utcstamp * INTERVAL '1' SECOND )
           AT TIME ZONE 'Europe/Amsterdam'
         AS DATE
       ) AS Amsterdam_Time
FROM   your_table


AMSTERDAM_TIME


2019-12-18 12:00:00


CREATE FUNCTION DATE_TO_UTCEPOCHTIME (
  dt IN DATE,
  tz IN VARCHAR2 DEFAULT 'UTC'
) RETURN NUMBER DETERMINISTIC
IS
BEGIN
  RETURN ROUND(
           ( CAST( FROM_TZ( dt, tz ) AT TIME ZONE 'UTC' AS DATE )
             - DATE '1970-01-01' )
           * 24 * 60 * 60
         );
END;
/
CREATE FUNCTION UTCEPOCHTIME_TO_DATE(
  utctime IN NUMBER,
  tz      IN VARCHAR2 DEFAULT 'UTC'
) RETURN DATE DETERMINISTIC
IS
BEGIN
  RETURN ( TIMESTAMP '1970-01-01 00:00:00 UTC' + utctime * INTERVAL '1' SECOND )
           AT TIME ZONE tz;
END;
/
SELECT utcepochtime_to_date( utcstamp, 'Europe/Amsterdam' )
FROM   your_table;


UTCEPOCHTIME_TO_DATE(UTCSTAMP,'EUROPE/AMSTERDAM')


2019-12-18 12:00:00


SELECT date_to_utcepochtime(
         DATE '2019-12-18' + INTERVAL '12:00:00' HOUR TO SECOND,
         'Europe/Amsterdam'
       ) AS utcepochtime
FROM   DUAL;


UTCEPOCHTIME


1576666800