title: Convert string to datetime
body: <p>I have a simple question - and it seems to be one asked many times before (see e.g <a href="https://stackoverflow.com/questions/466345/converting-string-into-datetime">here</a>, <a href="https://stackoverflow.com/questions/10624937/convert-datetime-object-to-a-string-of-date-only-in-python">here</a>, and <a href="https://stackoverflow.com/questions/19068269/how-to-convert-a-string-date-into-datetime-format-in-python">here</a>). Nonetheless, I can't solve it. </p>
  
  <p>I got a pandas dataframe I read from a csv file. It contains a column with the name <code>start-plan</code> with strings in the format <code>'05-04-2017'</code> (April 5th, 2017). As far as I understand, it is a European datetime in the form <code>%d-%m-%Y</code>. 
  Here  is what I do:</p>
  
  <pre><code>df = pd.read_csv('activities.csv')
  </code></pre>
  
  <p>This is what dataframe head looks like:</p>
  
  <pre><code>print(df.head())
  
         start-plan start-actual    end-plan  end-actual  user  late
  0  12-01-2017   16-01-2017  11-02-2017  10-02-2017     1     0
  1  11-05-2017   15-05-2017  10-06-2017  18-06-2017     2     1
  2  20-08-2017   20-08-2017  19-09-2017  05-10-2017     3     1
  3  10-12-2017   10-12-2017  09-01-2018  08-01-2018     1     0
  4  25-04-2017   25-04-2017  25-05-2017  26-05-2017     4     0
  </code></pre>
  
  <p>I try to convert the colums like this:</p>
  
  <pre><code>pd.to_datetime(pd.Series('start-plan'), format='%d-%m-%y')
  </code></pre>
  
  <p>I get an error stating that <code>time data 'start-plan' does not match format '%d-%M-%Y' (match)</code></p>
  
  <p>What am I doing wrong? Moreover, I have several columns in the same format that I would like to convert. Is there a possibility to convert them all at once?</p>