title: Wrong date displayed when converting in pandas
body: <p>I do not know why this snippet is producing 01/01/1970 for all the dates when I am sure this is not accurate.</p>
  
  <p>My Original File:</p>
  
  <pre><code>Appreciation Start              Funding Date
      41266                          41432
      41266                          41715
      41266                          41533
      41266                          41505
      41266                          41444
      41550                          41513
      41550                          41451
  </code></pre>
  
  <p>My snippet:</p>
  
  <pre><code>import pandas as pd
  
  df['Funding Date'] = pd.to_datetime(df['Funding Date']).apply(lambda x: x.strftime('%m/%d/%Y')if not pd.isnull(x) else '')
  df['Appreciation Start'] = pd.to_datetime(df['Appreciation Start']).apply(lambda x: x.strftime('%m/%d/%Y')if not pd.isnull(x) else '')
  
  df.to_excel('new.xlsx')
  </code></pre>
  
  <p>In Excel:</p>
  
  <pre><code>Appreciation Start              Funding Date
      01/01/1970                   01/01/1970
      01/01/1970                   01/01/1970
      01/01/1970                   01/01/1970
      ..........                ..............
  </code></pre>
  
  <p>How can I fix this?</p>