title: Convert class 'pandas.tslib.Timedelta' to string when export to excel
body: <p>Initial dataframe:</p>
  
  <pre><code>arrivalTime        
  0   2016-01-12 06:35:42
  2   2016-01-12 06:54:02
  3   2016-01-12 07:01:43
  4   2016-01-12 07:02:28
  5   2016-01-12 07:12:29
  6   2016-01-12 07:18:41 
  </code></pre>
  
  <p>On data I apply this function:</p>
  
  <pre><code>def function(df):
      df['arrivalTime_cal'] = pd.to_datetime(df['arrivalTime'], format='%Y-%m-%d %H:%M:%S')
      df['diff_time'] = df['arrivalTime_cal'].diff().fillna(0)
      del df['arrivalTime_cal']
      return df
  </code></pre>
  
  <p>and I get these results (corrects in ipython):</p>
  
  <pre><code>diff_time  
  0  00:00:00  
  1  00:04:37  
  2  00:13:43  
  3  00:07:41  
  4  00:00:45
  </code></pre>
  
  <p>when export to excel that result change format:</p>
  
  <pre><code>arrivalTime             diff_time
  0   2016-01-12 06:35:42 0
  1   2016-01-12 06:40:19 0,003206019
  2   2016-01-12 06:54:02 0,009525463
  3   2016-01-12 07:01:43 0,005335648
  4   2016-01-12 07:02:28 0,000520833
  </code></pre>
  
  <p>How do I keep the string format in Excel?</p>
  
  <p>Thank you in advance</p>