Hello riccarddo,
Please use the following:
WITH StarDate (firstday) as
(
SELECT d - DAYOFYEAR(d) DAYS + ((QUARTER(d) - 1) * 3) MONTHS - 3 MONTHS + 1 DAY
FROM (VALUES DATE('2017-04-21')) as D(d)
)
SELECT * FROM StarDate
This works in DB2 LUW.
Regards
Ondrej