title: Ignore NaN values when searching for a minimum datetime
body: <p>I have the following piece of code:</p>
  
  <pre><code>if isinstance(df["col1"][0], datetime.datetime):
     min_dt = min(df["col1"].values)
  </code></pre>
  
  <p>It searches for a minimum date value in a column <code>col1</code>. However, it checks the data type of the first rows, while there might be empty values in the rest of rows. How can I ignore empty values when searching for a minimum date value in a column?</p>
  
  <p>Now I get this error:</p>
  
  <pre><code>TypeError: '&lt;' not supported between instances of 'datetime.datetime' and 'str'
  </code></pre>