title: Unknown string format on pd.to_datetime
body: <p>I have a data set with a column date like this:</p>
  
  <pre><code>cod           date              value 
  0   1O8        2015-01-01 00:00:00    2.1
  1   1O8        2015-01-01 01:00:00    2.3
  2   1O8        2015-01-01 02:00:00    3.5
  3   1O8        2015-01-01 03:00:00    4.5
  4   1O8        2015-01-01 04:00:00    4.4
  5   1O8        2015-01-01 05:00:00    3.2
  6   1O9        2015-01-01 00:00:00    1.4
  7   1O9        2015-01-01 01:00:00    8.6
  8   1O9        2015-01-01 02:00:00    3.3
  10  1O9        2015-01-01 03:00:00    1.5
  11  1O9        2015-01-01 04:00:00    2.4
  12  1O9        2015-01-01 05:00:00    7.2
  </code></pre>
  
  <p>The <code>dtypes</code> of column date is an object, for apply some function after I need to change the date column type to datatime. I try a diffrent solution like:</p>
  
  <pre><code>pd.to_datetime(df['date'], errors='raise', format ='%Y-%m-%d HH:mm:ss')
  pd.to_datetime(df['date'], errors='coerce', format ='%Y-%m-%d HH:mm:ss')
  df['date'].apply(pd.to_datetime, format ='%Y-%m-%d HH:mm:ss')
  </code></pre>
  
  <p>But the error is only the same: </p>
  
  <pre><code>TypeError: Unrecognized value type: &lt;class 'str'&gt;
  ValueError: Unknown string format
  </code></pre>
  
  <p>The straight thing is that if I apply te function to a sample of data set, the function respond correctly, but if I apply it to all data set exit the error. In the data there isn missing value and the dtype is the same for all value. </p>
  
  <p>How I can fix this error?</p>