Time zone conversion between UTC and local time + possibly daylight saving - sql

Question

I'm struggling to deal with time zone and daylight saving when querying SAP HANA. The datetime stamp is in the form of NVARCHAR, eg 20210304132500 YYYYMMDDHHMISS in UTC, which means local time is 14:25:00(GMT +01:00) but my query returns 13:25:00 (UTC). How do I edit my results to match local time? Sample query below if that helps.
SELECT DATE_TIME,LOCATION,PART_NUMB
FROM "PUBLIC"."internal.sap.datamodel::ACTIVITY"
WHERE SUBSTRING(DATE_TIME,9,2) IN ('08','11')`
The desired result is local date_time in any format.

Answer 1

HANA comes with timezone conversion functions (UTCTOLOCAL) that can perform the necessary calculations.
These functions require that the data/time input is in either SQL date/time format or that it can be converted to that. They also require that the timezone data has been set up and maintained in the HANA DB. This is the actual information about which timezone has which offsets and daylight saving begin and end times.
For your example, it may make sense to expose the DATE_TIME as a type converted field DATE_TIME_UTC that is already in sql-date time:
to_seconddate (DATE_TIME, 'YYYYMMDDHHMISS') as DATE_TIME_UTC
With this conversion done, you can convert the timezone like this:
UTCTOLOCAL (DATE_TIME_UTC, 'Berlin', 'platform') as LOCAL_DATE_TIME
Note, that the target time zone name may be something like "GMT+1" but this is really just a name and not a calculation instruction. If "GMT+1" is not found in the list of timezone conversions, HANA won't just add an hour - it won't perform the calculation.
With this data type and timezone conversion done, you could have a WHERE clause like this:
WHERE
HOUR(LOCAL_DATE_TIME) IN (8, 11)
This order of transformations (data type -> time zone -> hour component) is of course rather expensive. It may be worthwhile to check whether the resulting query performance is satisfactory on realistic data volume.
Also important to note is that time zone conversion only works on complete date-time information, not just the time. That is to say, if the date is unknown, it cannot be determined which offset rule between two time zone applies. So, simply separating the hours and date components won't help in this case.
Finally, I've written quite a bit about handling date, time, and time zones in HANA, you may want to have a look at that:
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