title: TypeError: int() argument must be a string, a bytes-like object or a number, not 'slice'
body: <p>I used <code>fbprophet</code> datasets for time series analysis. The dataset has two columns, <code>date</code> and <code>y</code>, respectively.</p>
  
  <pre><code>  date                 y
  January 01, 1992      146376
  February 01, 1992     147079 
  March 01, 1992        159336   
  April 01, 1992        163669
  May 01, 1992          170068       
  
  
    date        y
  01/01/92    146376
  01/02/92    147079
  01/03/92    159336
  01/04/92    163669
  01/05/92    170068
  </code></pre>
  
  <p>I first changed the date into datetime format by using <code>pd.to_datetime</code> and then fit the model <code>model = Prophet().fit(df)</code>. However, the result keeps showing me <code>TypeError: int() argument must be a string, a bytes-like object or a number, not 'slice'</code>. Is there anyway to fix this problem? </p>
  
  <p>Here's my code,</p>
  
  <pre><code>df.date = pd.to_datetime(df.date)
  df['date'] = df['date'].dt.strftime('%Y-%m-%d')
  model = Prophet()
  model.fit(df)
  </code></pre>
  
  <p>When I run the <code>model.fit(df)</code>the <code>TypeError</code>I mentioned above shows up.</p>