Search notes:

ORA-01839: date not valid for month specified

April has only 30 days, the following statement throws ORA-01839: date not valid for month specified:

select
   to_date('2021-04-31', 'yyyy-mm-dd') hmmm
from
   dual;

NUMTOYMINTERVAL

The following statement runs without problems …

select
   date '2021-02-01' + numtoyminterval(1, 'month')
from
   dual;

… but this one throws ORA-01839: date not valid for month specified

select
   date '2021-01-31' + numtoyminterval(1, 'month')
from
   dual;