Oracle 21c

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ALTER SESSION SET NLS_TIMESTAMP_FORMAT = 'YYYY-MM-DD HH24:MI:SS.FF'
ALTER SESSION SET NLS_TIMESTAMP_TZ_FORMAT = 'YYYY-MM-DD HH24:MI:SS.FF TZH:TZM'
SELECT TIMESTAMP '1970-01-01 00:00:00.000 UTC'
         + NUMTODSINTERVAL( 1483228800000 / 1000, 'SECOND' )
         AS TIME
FROM   DUAL


TIME


2017-01-01 00:00:00.000000000 +00:00


CREATE OR REPLACE PACKAGE time_utils
IS
  FUNCTION milliseconds_since_epoch(
    in_datetime  IN TIMESTAMP,
    in_epoch     IN TIMESTAMP DEFAULT TIMESTAMP '1970-01-01 00:00:00'
  ) RETURN NUMBER;

  FUNCTION milliseconds_epoch_to_ts (
    in_milliseconds IN NUMBER,
    in_epoch        IN TIMESTAMP DEFAULT TIMESTAMP '1970-01-01 00:00:00'
  ) RETURN TIMESTAMP;
END;
/
CREATE OR REPLACE PACKAGE BODY time_utils
IS
  -- List of the seconds immediately following leap seconds:
  leap_seconds CONSTANT SYS.ODCIDATELIST := SYS.ODCIDATELIST(
      DATE '1972-07-01',
      DATE '1973-01-01',
      DATE '1974-01-01',
      DATE '1975-01-01',
      DATE '1976-01-01',
      DATE '1977-01-01',
      DATE '1978-01-01',
      DATE '1979-01-01',
      DATE '1980-01-01',
      DATE '1981-07-01',
      DATE '1982-07-01',
      DATE '1983-07-01',
      DATE '1985-07-01',
      DATE '1988-01-01',
      DATE '1990-01-01',
      DATE '1991-01-01',
      DATE '1992-07-01',
      DATE '1993-07-01',
      DATE '1994-07-01',
      DATE '1996-01-01',
      DATE '1997-07-01',
      DATE '1999-01-01',
      DATE '2006-01-01',
      DATE '2009-01-01',
      DATE '2012-07-01',
      DATE '2015-07-01',
      DATE '2016-01-01'
    );

  HOURS_PER_DAY           CONSTANT BINARY_INTEGER := 24;
  MINUTES_PER_HOUR        CONSTANT BINARY_INTEGER := 60;
  SECONDS_PER_MINUTE      CONSTANT BINARY_INTEGER := 60;
  MILLISECONDS_PER_SECOND CONSTANT BINARY_INTEGER := 1000;

  MINUTES_PER_DAY         CONSTANT BINARY_INTEGER := HOURS_PER_DAY   * MINUTES_PER_HOUR;
  SECONDS_PER_DAY         CONSTANT BINARY_INTEGER := MINUTES_PER_DAY * SECONDS_PER_MINUTE;

  MILLISECONDS_PER_MINUTE CONSTANT BINARY_INTEGER := SECONDS_PER_MINUTE * MILLISECONDS_PER_SECOND;
  MILLISECONDS_PER_HOUR   CONSTANT BINARY_INTEGER := MINUTES_PER_HOUR   * MILLISECONDS_PER_MINUTE;
  MILLISECONDS_PER_DAY    CONSTANT BINARY_INTEGER := HOURS_PER_DAY      * MILLISECONDS_PER_HOUR;

  FUNCTION milliseconds_since_epoch(
    in_datetime  IN TIMESTAMP,
    in_epoch     IN TIMESTAMP DEFAULT TIMESTAMP '1970-01-01 00:00:00'
  ) RETURN NUMBER
  IS
    p_leap_milliseconds BINARY_INTEGER := 0;
    p_diff              INTERVAL DAY(9) TO SECOND(3);
  BEGIN
    IF in_datetime IS NULL OR in_epoch IS NULL THEN
      RETURN NULL;
    END IF;

    p_diff := in_datetime - in_epoch;

    IF in_datetime >= in_epoch THEN
      FOR i IN 1 .. leap_seconds.COUNT LOOP
        EXIT WHEN in_datetime < leap_seconds(i);
        IF in_epoch < leap_seconds(i) THEN
          p_leap_milliseconds := p_leap_milliseconds + MILLISECONDS_PER_SECOND;
        END IF;
      END LOOP;
    ELSE
      FOR i IN REVERSE 1 .. leap_seconds.COUNT LOOP
        EXIT WHEN in_datetime > leap_seconds(i);
        IF in_epoch > leap_seconds(i) THEN
          p_leap_milliseconds := p_leap_milliseconds - MILLISECONDS_PER_SECOND;
        END IF;
      END LOOP;
    END IF;

    RETURN   MILLISECONDS_PER_SECOND * EXTRACT( SECOND FROM p_diff )
           + MILLISECONDS_PER_MINUTE * EXTRACT( MINUTE FROM p_diff )
           + MILLISECONDS_PER_HOUR   * EXTRACT( HOUR   FROM p_diff )
           + MILLISECONDS_PER_DAY    * EXTRACT( DAY    FROM p_diff )
           + p_leap_milliseconds;
  END milliseconds_since_epoch;

  FUNCTION milliseconds_epoch_to_ts(
    in_milliseconds IN NUMBER,
    in_epoch        IN TIMESTAMP DEFAULT TIMESTAMP '1970-01-01 00:00:00'
  ) RETURN TIMESTAMP
  IS
    p_datetime TIMESTAMP;
  BEGIN
    IF in_milliseconds IS NULL OR in_epoch IS NULL THEN
      RETURN NULL;
    END IF;

    p_datetime := in_epoch
        + NUMTODSINTERVAL( in_milliseconds / MILLISECONDS_PER_SECOND, 'SECOND' );

    IF p_datetime >= in_epoch THEN
      FOR i IN 1 .. leap_seconds.COUNT LOOP
        EXIT WHEN p_datetime < leap_seconds(i);
        IF in_epoch < leap_seconds(i) THEN
          p_datetime := p_datetime - INTERVAL '1' SECOND;
        END IF;
      END LOOP;
    ELSE
      FOR i IN REVERSE 1 .. leap_seconds.COUNT LOOP
        EXIT WHEN p_datetime > leap_seconds(i);
        IF in_epoch > leap_seconds(i) THEN
          p_datetime := p_datetime + INTERVAL '1' SECOND;
        END IF;
      END LOOP;
    END IF;

    RETURN p_datetime;
  END milliseconds_epoch_to_ts;
END;
/
SELECT * FROM USER_ERRORS;
SELECT TIME_UTILS.milliseconds_epoch_to_ts(
         in_milliseconds => 1483228800000,
         in_epoch        => TIMESTAMP '1970-01-01 00:00:00.000'
       ) AS time
FROM DUAL;


TIME


2016-12-31 23:59:33.000000000