using oracle to_date function without a format mask

Question

this may be a dumb question however.. I have these two selects: 1st one returns a result, while 2nd gives me a not a valid month error. I can't seem to figure out why..

select to_date('09-12-14','dd-mm-rr') - to_date('01/january/2007') from dual;
select to_date('09-12-14','dd-mm-rr') - to_date('01-01-2007') from dual;

Tnx in advance

Answer 1

When you don't specify a format mask, Oracle will use the NLS_DATE_FORMAT setting (and some predefined masks) to determine the date time format mask.

Apparently your server settings support the first format, and not the second. Therefore it is better to always specify your format mask.