title: How to count total days in pandas dataframe
body: <p>I have a df column with dates and hours / minutes:</p>
  
  <pre><code>0   2019-09-13 06:00:00
  1   2019-09-13 06:05:00
  2   2019-09-13 06:10:00
  3   2019-09-13 06:15:00
  4   2019-09-13 06:20:00
  Name: Date, dtype: datetime64[ns]
  </code></pre>
  
  <p>I need to count how many days the dataframe contains.</p>
  
  <p>I tried it like this:</p>
  
  <pre><code>sample_length = len(df.groupby(df['Date'].dt.date).first())
  </code></pre>
  
  <p>and</p>
  
  <pre><code>sample_length = len(df.groupby(df['Date'].dt.date))
  </code></pre>
  
  <p>But the number I get seems wrong. Do you know another method of counting the days?</p>