Oracle 11g Release 2

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CREATE TABLE table_name ( start_time TIMESTAMP WITH LOCAL TIME ZONE );
ALTER SESSION SET TIME_ZONE = 'PST';
ALTER SESSION SET NLS_TIMESTAMP_FORMAT = 'YYYY-MM-DD"T"HH24:MI:SS.FF3';
ALTER SESSION SET NLS_TIMESTAMP_TZ_FORMAT = 'YYYY-MM-DD"T"HH24:MI:SS.FF3 TZH:TZM';
INSERT INTO table_name ( start_time )
  SELECT TIMESTAMP '2019-09-17 00:00:00'        FROM DUAL UNION ALL
  SELECT TIMESTAMP '2019-09-17 00:00:00 +01:00' FROM DUAL
  
2 rows affected

ALTER SESSION SET TIME_ZONE = 'UTC';
SELECT start_time,
       start_time AT TIME ZONE 'PST' AS start_time_pst,
       start_time AT LOCAL           AS start_time_local
FROM   table_name;


START_TIME
START_TIME_PST
START_TIME_LOCAL


2019-09-17T07:00:00.000
2019-09-17T00:00:00.000 -07:00
2019-09-17T07:00:00.000 +00:00

2019-09-16T23:00:00.000
2019-09-16T16:00:00.000 -07:00
2019-09-16T23:00:00.000 +00:00


ALTER SESSION SET TIME_ZONE = 'EST';
SELECT start_time,
       start_time AT TIME ZONE 'PST' AS start_time_pst,
       start_time AT LOCAL           AS start_time_local
FROM   table_name;


START_TIME
START_TIME_PST
START_TIME_LOCAL


2019-09-17T02:00:00.000
2019-09-17T00:00:00.000 -07:00
2019-09-17T02:00:00.000 -05:00

2019-09-16T18:00:00.000
2019-09-16T16:00:00.000 -07:00
2019-09-16T18:00:00.000 -05:00

START_TIME	START_TIME_PST	START_TIME_LOCAL
2019-09-17T07:00:00.000	2019-09-17T00:00:00.000 -07:00	2019-09-17T07:00:00.000 +00:00
2019-09-16T23:00:00.000	2019-09-16T16:00:00.000 -07:00	2019-09-16T23:00:00.000 +00:00

START_TIME	START_TIME_PST	START_TIME_LOCAL
2019-09-17T02:00:00.000	2019-09-17T00:00:00.000 -07:00	2019-09-17T02:00:00.000 -05:00
2019-09-16T18:00:00.000	2019-09-16T16:00:00.000 -07:00	2019-09-16T18:00:00.000 -05:00