将日期从 UTC 时区转换为 PST 时区（包括 DST 因素）

Question

I want to convert a date in UTC timezone to PST timezone. The NEW_TIME doesn't take into account the DST (Daylight Saving Time) factor, so I tried using CAST and TO_TIMESTAMP_TZ functions but both gave incorrect time ( difference of around 5 hrs 30 min). Not sure why.

SELECT TO_CHAR(TO_TIMESTAMP_TZ(max(end_date) AT TIME ZONE 'PST')
      ,'DD-MON-YYYY HH24:MI:SS')
FROM table1
WHERE NAME= 'FIRST';

SELECT TO_CHAR(CAST((max(end_date) AT TIME ZONE 'PST') AS DATE )
      ,'DD-MON-YYYY HH24:MI:SS')
FROM table1
WHERE NAME= 'FIRST';

Instead of 'PST' I also tried using 'US/PACIFIC', but it too gave the same result.

max(end_date) is: 2021-03-15 07:17:16 (in UTC)

The query is returning (incorrect time returned): 14-MAR-2021 18:47:16

The time that it should (correct time expected): 15-MAR-2021 00:17:16

Can anyone please help in correcting my query or any other function that can convert the date from UTC to PST time zone (keeping the DST factor in mind).

Answer 1

You have 6 hours time difference, so I'm going to assume that you are in the same time zone as Asia/Dacca and have set up my session using:

ALTER SESSION SET TIME_ZONE='Asia/Dacca';

Now, if I create table1 with the data type TIMESTAMP WITH TIME ZONE :

CREATE TABLE table1 (
  name     VARCHAR2(20),
  end_date TIMESTAMP WITH TIME ZONE
);

INSERT INTO table1 ( name, end_date ) VALUES ( 'FIRST', TIMESTAMP '2021-03-15 07:17:16 UTC' );

Then your query (you do not need to use TO_TIMESTAMP_TZ on a column that is already a TIMESTAMP WITH TIME ZONE column):

SELECT TO_CHAR(
         max(end_date) AT TIME ZONE 'PST',
         'DD-MON-YYYY HH24:MI:SS'
       ) AS pst_end_date
FROM   table1
WHERE  NAME = 'FIRST';

Outputs:

 |  PST_END_DATE |  |:------------------- |  |  15-MAR-2021 00:17:16 | 

and works!

---

However, if you store end_date using a TIMESTAMP (without time zone):

CREATE TABLE table1 (
  name     VARCHAR2(20),
  end_date TIMESTAMP
);

INSERT INTO table1 ( name, end_date ) VALUES ( 'FIRST', TIMESTAMP '2021-03-15 07:17:16' );

Then:

SELECT TO_CHAR(
         max(end_date) AT TIME ZONE 'PST',
         'DD-MON-YYYY HH24:MI:SS'
       ) AS pst_end_date
FROM   table1
WHERE  NAME = 'FIRST';

Outputs:

 |  PST_END_DATE |  |:------------------- |  |  14-MAR-2021 18:17:16 | 

Which replicates your issue.

This is because the database does not know the time zone of the data and will implicitly assume that it is the same as the database/session time zone and we've set that to Asia/Dacca and not UTC . Instead we need to explicitly tell the database to use the UTC time zone for the conversion:

SELECT TO_CHAR(
         FROM_TZ(max(end_date), 'UTC') AT TIME ZONE 'PST',
         'DD-MON-YYYY HH24:MI:SS'
       ) AS pst_end_date
FROM   table1
WHERE  NAME = 'FIRST';

Which outputs:

 |  PST_END_DATE |  |:------------------- |  |  15-MAR-2021 00:17:16 | 

---

If your column has the DATE data type:

CREATE TABLE table1 (
  name     VARCHAR2(20),
  end_date DATE
);

INSERT INTO table1 ( name, end_date ) VALUES ( 'FIRST', TIMESTAMP '2021-03-15 07:17:16' );

Then you can use the same query with an added CAST :

SELECT TO_CHAR(
         FROM_TZ(CAST(max(end_date) AS TIMESTAMP), 'UTC') AT TIME ZONE 'PST',
         'DD-MON-YYYY HH24:MI:SS'
       ) AS pst_end_date
FROM   table1
WHERE  NAME = 'FIRST';

db<>fiddle here