Oracle 18c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
SELECT TO_DATE( TO_CHAR(dt, 'YYYY') || '-01-01', 'YYYY-MM-DD' )
         + INTERVAL '7' DAY * (TO_CHAR(dt, 'WW') - 1) AS week_start,
       TRUNC(dt, 'WW') AS trunc_week
FROM   (
  SELECT TO_DATE('1970-01-01 07:00:00', 'YYYY-MM-DD HH24:MI:SS') AS dt
  FROM   DUAL
);


WEEK_START
TRUNC_WEEK


1970-01-01 00:00:00
1970-01-01 00:00:00


SELECT TRUNC( TO_DATE( TO_CHAR(dt, 'IYYY') || '-01-04', 'YYYY-MM-DD' ), 'IW')
         + INTERVAL '7' DAY * (TO_CHAR(dt, 'IW') - 1) AS iso_week_start,
       TRUNC(dt, 'IW') AS trunc_iso_week
FROM   (
  SELECT TO_DATE('1970-01-01 07:00:00', 'YYYY-MM-DD HH24:MI:SS') AS dt
  FROM   DUAL
);


ISO_WEEK_START
TRUNC_ISO_WEEK


1969-12-29 00:00:00
1969-12-29 00:00:00