title: Statsmodels ARIMA, Value warning, date index and associated frequency information
body: <p>I am currently building ARIMA models in python and has come across this error which I dont really understand when looking through the error code. I am curious if it affects the parameters in the model and would really appreciate if someone could advice on the issue, probably suggest a fix.</p>
  <p>A date index has been provided, but it has no associated frequency information and so will be ignored when e.g. forecasting.</p>
  <p><a href="https://i.stack.imgur.com/sNZjd.png" rel="nofollow noreferrer">https://i.stack.imgur.com/sNZjd.png</a></p>
  <p>The code in the error documentation in github goes as:</p>
  <pre><code>date_index = isinstance(index, (DatetimeIndex, PeriodIndex))
  
      if date_index and not has_freq:
          warnings.warn('A date index has been provided, but it has no'
                        ' associated frequency information and so will be'
                        ' ignored when e.g. forecasting.', ValueWarning)
  </code></pre>
  <p>Here is my dataframe. I have the date time set as my dataframe index.</p>
  <p><a href="https://i.stack.imgur.com/rlkd0.png" rel="nofollow noreferrer">https://i.stack.imgur.com/rlkd0.png</a></p>
  <p>Just to be clear, the code is still executable and the result summary can be printed.</p>