Oracle 18c

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create table mytable (field1, field2, field3, date_one, date_two) as
select 'a', 'b', 'c', '29-01-2019 20:22:08', date '2019-01-25' from dual
union all
select 'a', 'b', 'c', '29-01-2019 20:22:08', date '2019-01-01' from dual
2 rows affected

select field1,field2,field3
from mytable
where trunc(to_date(DATE_ONE, 'DD-MM-YYYY HH24:MI:SS'))
  - trunc(to_date(DATE_TWO, 'DD-MM-YYYY')) > interval '15' day;
ORA-00932: inconsistent datatypes: expected NUMBER got INTERVAL DAY TO SECOND

select field1,field2,field3
from mytable
where trunc(to_date(DATE_ONE, 'DD-MM-YYYY HH24:MI:SS')) - trunc(DATE_TWO) > interval '15' day;
ORA-00932: inconsistent datatypes: expected NUMBER got INTERVAL DAY TO SECOND

-- just to show the difference is a number
select field1,field2,field3,
  trunc(to_date(DATE_ONE, 'DD-MM-YYYY HH24:MI:SS')) - trunc(DATE_TWO) as diff1,
  (trunc(to_date(DATE_ONE, 'DD-MM-YYYY HH24:MI:SS')) - trunc(DATE_TWO)) * interval '1' day as diff2
from mytable;


FIELD1
FIELD2
FIELD3
DIFF1
DIFF2


a
b
c
4
+000000004 00:00:00.000000000

a
b
c
28
+000000028 00:00:00.000000000


select field1,field2,field3
from mytable
where (trunc(to_date(DATE_ONE, 'DD-MM-YYYY HH24:MI:SS')) - trunc(DATE_TWO)) * interval '1' day > interval '15' day;


FIELD1
FIELD2
FIELD3


a
b
c


select field1,field2,field3
from mytable
where trunc(to_date(DATE_ONE, 'DD-MM-YYYY HH24:MI:SS'))
  - trunc(DATE_TWO) > 15;


FIELD1
FIELD2
FIELD3


a
b
c

FIELD1	FIELD2	FIELD3	DIFF1	DIFF2
a	b	c	4	+000000004 00:00:00.000000000
a	b	c	28	+000000028 00:00:00.000000000