Oracle 18c

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alter session set nls_date_format = 'YYYY-MM-DD';
SELECT
  TRUNC(SYSDATE) AS today,
  NEXT_DAY(TRUNC(SYSDATE), 'MON') as next_mon,
  NEXT_DAY(TRUNC(SYSDATE), 'MON') - TRUNC(SYSDATE) as days_a,
  NEXT_DAY(TRUNC(SYSDATE), 'MON') - TRUNC(SYSDATE) - 3 as days_b,
  GREATEST(NEXT_DAY(TRUNC(SYSDATE), 'MON') - TRUNC(SYSDATE) - 3, 0) as result
FROM DUAL


TODAY
NEXT_MON
DAYS_A
DAYS_B
RESULT


2020-06-29
2020-07-06
7
4
4


SELECT
  TRUNC(SYSDATE) AS today,
  NEXT_DAY(TRUNC(SYSDATE) - 1, 'MON') as next_mon,
  NEXT_DAY(TRUNC(SYSDATE) - 1, 'MON') - TRUNC(SYSDATE) as days_a,
  NEXT_DAY(TRUNC(SYSDATE) - 1, 'MON') - TRUNC(SYSDATE) - 3 as days_b,
  GREATEST(NEXT_DAY(TRUNC(SYSDATE) - 1, 'MON') - TRUNC(SYSDATE) - 3, 0) as result
FROM DUAL


TODAY
NEXT_MON
DAYS_A
DAYS_B
RESULT


2020-06-29
2020-06-29
0
-3
0