title: Pandas df.index.round('D') for 24hr time
body: <p>Given a dataframe with 24hr datetime as index, rounding the datetime as days seems to increment the 'day' after 1200 hrs.</p>
  
  <p>Here's a modified code-snippet from another example that generates random numbers.</p>
  
  <p>Please note the output datetime index 'day' compared to the rounded 'day'.</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  from datetime import datetime, timedelta
  
  date_today = datetime.now()
  days = pd.date_range(date_today, date_today + timedelta(1), freq='H')
  
  np.random.seed(seed=1111)
  data = np.random.randint(1, high=100, size=len(days))
  df = pd.DataFrame({'test': days, 'col2': data})
  df = df.set_index('test')
  
  df['datetime_rounded']  = df.index.round('D')
  print(df)
  ...
  2018-10-12 10:19:29.907421    43       2018-10-12
  2018-10-12 11:19:29.907421    77       2018-10-12
  2018-10-12 12:19:29.907421    95       2018-10-13
  2018-10-12 13:19:29.907421    79       2018-10-13
  </code></pre>
  
  <p>How can I ensure that the rounded 'day' ends at the end of the Datetime index 'day'?</p>
  
  <p>Thanks,</p>