title: Create monthly pandas DatetimeIndex
body: <p>I have a dataframe with 12 values that I want to convert to DatetimeIndex type</p>
  
  <pre><code>months = df['date'] #e.g. '2016-04-01'
  idx = pd.date_range(months[0], months[11], freq='M')
  </code></pre>
  
  <p>However, the resulting DatetimeIndex length is only 11, which should be 12
  Not sure if is it because of the freq, which shifts date to the month end?</p>