Determine if spring or fall and adjust time if DST or not - oracle

Question

I have a query like this:
SELECT
to_date(to_char((from_tz(to_timestamp(to_char(my_column, 'YYYY-MM-DD HH:MI:SS PM'), 'YYYY-MM-DD HH:MI:SS PM') ,'America/Edmonton')at time zone 'America/Vancouver'),'YYYY-MM-DD'),'YYYY-MM-DD') as Date_Column
FROM
my_table
It converts the time between MST to PST. This works most of the time except when daylight savings time occurs. Then a situation happens when in the clock moves forward an hour, and then since an hour is lost (for example 1:59 to the 3:00 am) then we are trying to convert a fictional time.
So I am aware of the issue, and I know Oracle will throw this error for that reason:
ORA-01878: specified field not found in datetime or interval
I've looked all over the Internet for a possible work around but can't really find anything that works. I just want to modify my query so that it can tell what time of year it is and either do the conversation when the time exists, or handle the time somehow when it does not exists. Perhaps add the extra hour when it's missing, and remove it when it's the other time of year.
Does anyone have a solution for this? Is this an impossible problem?

Answer 1

Instead of all the conversions you are using, just use AT TIME ZONE. You may need to take extra care depending on the data type of your column. The AT TIME ZONE functionality will always take into account daylights savings time.
If your column is a TIMESTAMP WITH TIME ZONE, that would be ideal
because you could just say AT TIME ZONE 'PST' and you would get the
time in PST.
If the column is the datatype TIMESTAMP, when you use AT TIME ZONE it
will convert from the time zone of the current session to whatever
you specify.
If your column is a DATE, you will need to convert it to a TIMESTAMP first, then you can use the AT TIME ZONE function. Converting the DATE to TIMESTAMP will make the time zone 0:00 offset so be sure to account for that.
Another handy trick is to use AT LOCAL. This will convert the timestamp to the time zone of the current session. This may be handy if you have users of your application that are in multiple time zones.
Example
I am in the time zone -04:00 (East coast US) which is why that appears as my time zone in the examples below.
WITH
my_table
AS
(SELECT TIMESTAMP '2021-01-01 8:00:00 -7:00' AS ts_with_tz,
TIMESTAMP '2021-01-01 8:00:00' AS ts,
DATE '2021-01-01' + (8 / 24) AS dt
FROM DUAL)
SELECT 1 AS example_num,
t.ts_with_tz,
t.ts,
t.dt
FROM my_table t
UNION ALL
SELECT 2 AS example_num,
t.ts_with_tz AT TIME ZONE 'PST',
t.ts AT TIME ZONE 'PST',
TO_TIMESTAMP (t.dt) AT TIME ZONE 'PST'
FROM my_table t
ORDER BY 1;
RESULT
EXAMPLE_NUM TS_WITH_TZ TS DT
______________ ______________________________________________________ ______________________________________________________ ______________________________________________________
1 01-JAN-21 08.00.00.000000000 AM -07:00 01-JAN-21 08.00.00.000000000 AM AMERICA/NEW_YORK 01-JAN-21 08.00.00.000000000 AM AMERICA/NEW_YORK
2 01-JAN-21 07.00.00.000000000 AM AMERICA/LOS_ANGELES 01-JAN-21 05.00.00.000000000 AM AMERICA/LOS_ANGELES 31-DEC-20 09.00.00.000000000 PM AMERICA/LOS_ANGELES