title: Pandas to_csv AmbiguousTimeError
body: <p>When exporting a dataframe (pandas version 17.0) with a time zone aware datetime index and daylight saving times, the to_csv method raises an AmbiguousTimeError time error. E.g.,</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  df = pd.DataFrame(data={"c1": np.random.randn(960)}, 
                    index=pd.date_range("2015-10-1", periods=960, freq="H", tz="Europe/Berlin"))
  df.to_csv("test.csv")
  </code></pre>
  
  <p>Leads to an error that daylight savings time cannot be inferred:</p>
  
  <blockquote>
    <p>AmbiguousTimeError: Cannot infer dst time from Timestamp('2015-10-25 02:00:00'), try using the 'ambiguous' argument</p>
  </blockquote>
  
  <p>As a workaround I currently convert the index to string before exporting:</p>
  
  <pre><code>df.index = df.index.map(str)
  </code></pre>
  
  <p>Is there a way to directly address the issue when calling the to_csv method?</p>
  
  <h2>Update1</h2>
  
  <pre><code>pd.show_versions()
  
  INSTALLED VERSIONS
  ------------------
  commit: None
  python: 2.7.9.final.0
  python-bits: 64
  OS: Windows
  OS-release: 8
  machine: AMD64
  processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
  byteorder: little
  LC_ALL: None
  LANG: None
  
  pandas: 0.17.0
  nose: 1.3.4
  pip: 7.1.2
  setuptools: 18.3.1
  Cython: None
  numpy: 1.10.0b1
  scipy: 0.14.0
  statsmodels: 0.6.1
  IPython: 3.2.1
  sphinx: 1.2.2
  patsy: 0.3.0
  dateutil: 2.4.2
  pytz: 2015.4
  blosc: None
  bottleneck: 0.8.0
  tables: 3.1.1
  numexpr: 2.4
  matplotlib: 1.4.3
  openpyxl: 1.8.6
  xlrd: 0.9.3
  xlwt: 0.7.5
  xlsxwriter: 0.5.7
  lxml: 3.3.5
  bs4: 4.3.2
  html5lib: 0.999
  httplib2: None
  apiclient: None
  sqlalchemy: 0.9.7
  pymysql: None
  psycopg2: 2.5.3 (dt dec pq3 ext)
  </code></pre>
  
  <h2>Update2</h2>
  
  <p>As an update to this, it seems that assigning a date range with daylight savings time as DateTimeIndex does not work at all and produces the ambiguous time error:</p>
  
  <pre><code>pd.DatetimeIndex(pd.date_range("2015-10-25 00:00", "2015-10-25 04:00", freq="H", tz="Europe/Berlin"))
  </code></pre>
  
  <p>Any hints what I am missing?</p>