title: pandas to_datetime gets different years with parameter #format or not
body: <p>Without format param:</p>
  
  <pre><code>pd.to_datetime(1)
  </code></pre>
  
  <p>output:</p>
  
  <pre><code>Timestamp('1970-01-01 00:00:00.000000001')
  </code></pre>
  
  <p>With format param:</p>
  
  <pre><code>pd.to_datetime(1, format="%H")
  </code></pre>
  
  <p>output:</p>
  
  <pre><code>Timestamp('1900-01-01 01:00:00')
  </code></pre>
  
  <p>The first output year is 1970, while the 2nd output is 1900.</p>
  
  <p>Anyone knows why?</p>