title: re-setting date only in pandas series datetime index
body: <p>I have a pandas series <code>myS</code></p>
  
  <pre><code>import pandas as pd
  </code></pre>
  
  <p>The index is a set of strings containing time only</p>
  
  <pre><code>myS.index
  
  Out[28]: 
  Index([u'12:00 AM', u'12:14 AM', u'12:18 AM', u'12:25 AM', u'12:26 AM',
         u'12:37 AM', u'12:41 AM', u'12:47 AM', u'12:55 AM', u'12:59 AM',
         ...
         u'11:00 PM', u'11:02 PM', u'11:09 PM', u'11:18 PM', u'11:25 PM',
         u'11:35 PM', u'11:42 PM', u'11:46 PM', u'11:50 PM', u'11:55 PM'],
        dtype='object', name=u'Time (CET)', length=169)
  </code></pre>
  
  <p>I can conveniently convert this to datetime correctly:</p>
  
  <pre><code>myS.index= pd.to_datetime(myS.index, format='%I:%M %p')
  </code></pre>
  
  <p>However, all the dates will be set to <code>1900-01-01</code></p>
  
  <pre><code> '1900-01-01 23:50:00', '1900-01-01 23:55:00'],
                dtype='datetime64[ns]',
  </code></pre>
  
  <p>If I have a datetime available, how can I reset all the dates of the index to a desired value while leaving intact the time?</p>