#1747 DATEADD does. not take DST into account

[Pavel Antoshin]

How to reproduce

SELECT

    TIMESTAMP ( '2026-03-10', '20:30:40' ) AT TIME ZONE 'Europe/Paris' ,
    DATEADD ( MONTH , 1 , TIMESTAMP ( '2026-03-10', '20:30:40' ) AT TIME ZONE 'Europe/Paris' )

 FROM "PUBLIC"."t"

Expected

Since the result of DATEADD is inside the DST period, it should have offset +02:00. Both the original value and the result of DATEADD ( MONTH , ... should have the same time.

Actual

The query returns:

2026-03-10T20:30:40+01:00   2026-04-10T20:30:40+01:00

The result of DATEADD is 1 hour earlier in the Europe/Paris time zone

[Fred Toussi]

This is not a bug. HSQLDB does not store the time zone as 'Europe/Paris'. It is immediately converted to the time offset of +01:00 hour. When you add one month to the timestamp, it keeps the original DST.