title: Pandas `period_range` gives strange results
body: <p>I want a pandas period range with 25 hours offset, and I saw there are two ways to do this (see <a href="https://github.com/AileenNielsen/TimeSeriesAnalysisWithPython/blob/master/1.%20Dates%20%26%20Times.ipynb" rel="nofollow">here</a>):</p>
  
  <p>The first way is to use <code>freq=25H</code>, which I tried, and gave me the right answer:</p>
  
  <pre><code>import pandas as pd
  pd.period_range(start='2016-01-01 10:00', freq = '25H', periods = 10)
  </code></pre>
  
  <p>and the result is </p>
  
  <pre><code>PeriodIndex(['2016-01-01 10:00', '2016-01-02 11:00', '2016-01-03 12:00',
               '2016-01-04 13:00', '2016-01-05 14:00', '2016-01-06 15:00',
               '2016-01-07 16:00', '2016-01-08 17:00', '2016-01-09 18:00',
               '2016-01-10 19:00'],
              dtype='int64', freq='25H')   
  </code></pre>
  
  <p>The second way, using <code>freq=1D1H</code>, however, gave me a rather strange result:</p>
  
  <pre><code>pd.period_range(start='2016-01-01 10:00', freq = '1D1H', periods = 10)
  </code></pre>
  
  <p>and I got</p>
  
  <pre><code> PeriodIndex(['1971-12-02 01:00', '1971-12-02 02:00', '1971-12-02 03:00',
                '1971-12-02 04:00', '1971-12-02 05:00', '1971-12-02 06:00',
                '1971-12-02 07:00', '1971-12-02 08:00', '1971-12-02 09:00',
                '1971-12-02 10:00'],
              dtype='int64', freq='25H')
  </code></pre>
  
  <p>So maybe <code>1D1H</code> is not a valid way to specify frequency? how did <code>1971</code> come up? (I also tried to use use <code>1D1H</code> as frequency for the <code>date_range()</code> method, which did yield the right answer.)</p>
  
  <pre><code>pd.date_range('2016-01-01 10:00', freq = '1D1H', periods = 10)
  DatetimeIndex(['2016-01-01 10:00:00', '2016-01-02 11:00:00',
                 '2016-01-03 12:00:00', '2016-01-04 13:00:00',
                 '2016-01-05 14:00:00', '2016-01-06 15:00:00',
                 '2016-01-07 16:00:00', '2016-01-08 17:00:00',
                 '2016-01-09 18:00:00', '2016-01-10 19:00:00'],
                dtype='datetime64[ns]', freq='25H')
  </code></pre>
  
  <p>EDIT: it appears that with <code>period_range()</code>, though <code>freq=1D1H</code> doesn't work, <code>freq=1H1D</code> does. The reason is still unknown.</p>
  
  <p>EDIT2: this has been identified as a bug, see the answer below.</p>