title: subtracting part of a column in a data frame
body: <p>I have a date column in a <code>dataframe</code> but its type is string :</p>
  
  <pre><code>palma.Created_at[1:5]
  </code></pre>
  
  <pre><code>- 1    2019-10-20 12:08:40 +0200
  - 2    2019-10-20 12:08:40 +0200
  - 3    2019-10-20 12:08:40 +0200
  - 4    2019-10-20 12:08:40 +0200
  - 5    2019-10-20 12:08:40 +0200
  - Name: Created_at, dtype: object
  </code></pre>
  
  <p>and I want to be just like : <code>2019-10-20</code>. So how to  convert it like this and should I convert it to date type or no?</p>