title: using oracle to_date function without a format mask
body: <p>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..</p>
  
  <pre><code>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;
  </code></pre>
  
  <p>Tnx in advance </p>