title: python 2.7 mktime gives different values of the same inputs
body: <p>The following code works well on Windows, the results in Linux[Ubuntu] is weird. 
  My python version is 2.7 </p>
  
  <pre><code>from datetime import datetime
  from datetime import timedelta
  import time
  
  def time_diff(date_str_1, date_str_2):
      time_stamp_1 = time.mktime(time.strptime(date_str_1, '%Y-%m-%d-%H-%M-%S'))
      time_stamp_2 = time.mktime(time.strptime(date_str_2, '%Y-%m-%d-%H-%M-%S'))
      print '==='
      print date_str_1,date_str_2
      print time_stamp_1,time_stamp_2
      delta1 = time_stamp_2 - time_stamp_1
      return delta1
  
  print time_diff('2015-11-01-01-00-00', '2015-11-01-01-30-00')
  print time_diff('2015-11-01-01-00-00', '2015-11-01-02-00-00')
  print time_diff('2015-11-01-01-00-00', '2015-11-01-02-30-00')
  print time_diff('2015-11-01-01-00-00', '2015-11-01-03-00-00')
  </code></pre>
  
  <h3>results in Windows</h3>
  
  <pre><code>===
  2015-11-01-01-00-00 2015-11-01-01-30-00
  1446310800.0 1446312600.0
  1800.0
  ===
  2015-11-01-01-00-00 2015-11-01-02-00-00
  1446310800.0 1446314400.0
  3600.0
  ===
  2015-11-01-01-00-00 2015-11-01-02-30-00
  1446310800.0 1446316200.0
  5400.0
  ===
  2015-11-01-01-00-00 2015-11-01-03-00-00
  1446310800.0 1446318000.0
  7200.0
  </code></pre>
  
  <h3>results in Linux</h3>
  
  <pre><code>===
  2015-11-01-01-00-00 2015-11-01-01-30-00
  1446364800.0 1446366600.0
  1800.0
  ===
  2015-11-01-01-00-00 2015-11-01-02-00-00
  1446364800.0 1446372000.0
  7200.0
  ===
  2015-11-01-01-00-00 2015-11-01-02-30-00
  1446368400.0 1446373800.0
  5400.0
  ===
  2015-11-01-01-00-00 2015-11-01-03-00-00
  1446368400.0 1446375600.0
  7200.0
  </code></pre>
  
  <p>on Linux
  '2015-11-01-01-00-00' sometimes is 1446364800.0  and then it becomes 1446368400.0 </p>
  
  <p>What's the reason? is it a bug?</p>