title: Issue while converting date to datetime format in pandas dataframe
body: <p>Here is the dataframe. I want the dates here in <code>'%Y-%m-%d %H:%M:%S'</code> format.</p>
  
  <pre><code>import pandas as pd
  df2 = pd.DataFrame([['2017-18','','','','','','','','','','','',''], ['COMPANIES', '01-APR-2017', '01-MAY-2017', '01-JUN-2017', 
                      '01-JULY-2017', '01-AUG-2017', '01-SEP-2017', '01-OCT-2017', '01-NOV-2017', '01-DEC-2017', '01-JAN-2018', '01-FEB-2018', '01-MAR-2018']])
  </code></pre>
  
  <p>I tried,</p>
  
  <pre><code>df2.iloc[1, 1:] = df2.iloc[1, 1:].str.replace("JULY", "JUL")
  df2.iloc[1, 1:] = df2.iloc[1, 1:].apply(pd.to_datetime, format = '%d-%b-%Y')
  </code></pre>
  
  <p>but, it gives: </p>
  
  <pre><code>          0                    1                    2                    3   \
  0    2017-18                                                                  
  1  COMPANIES  1491004800000000000  1493596800000000000  1496275200000000000   
  
                      4                    5                    6   \
  0                                                                  
  1  1498867200000000000  1501545600000000000  1504224000000000000   
  
                      7                    8                    9   \
  0                                                                  
  1  1506816000000000000  1509494400000000000  1512086400000000000   
  
                      10                   11                   12  
  0                                                                 
  1  1514764800000000000  1517443200000000000  1519862400000000000  
  </code></pre>
  
  <p>Am I missing something? Is there any other way to achieve the dates in required format?</p>
  
  <p>I even tried:</p>
  
  <pre><code>for i in df2.iloc[1, 1:]:
      i = datetime.datetime.fromtimestamp(int(i)).strftime('%Y-%m-%d %H:%M:%S')
  </code></pre>
  
  <p>but gives a <code>ValueError: timestamp out of range for platform localtime()/gmtime() function</code></p>