title: Add days to dates in dataframe
body: <p>I am stymied at the moment.  I am sure that I am missing something simple, but how do you move a series of dates forward by x units?  In my more specific case I want to add 180 days to a date series within a dataframe.</p>
  
  <p>Here is what I have so far:</p>
  
  <pre><code>import pandas, numpy, StringIO, datetime
  
  
  txt = '''ID,DATE
  002691c9cec109e64558848f1358ac16,2003-08-13 00:00:00
  002691c9cec109e64558848f1358ac16,2003-08-13 00:00:00
  0088f218a1f00e0fe1b94919dc68ec33,2006-05-07 00:00:00
  0088f218a1f00e0fe1b94919dc68ec33,2006-06-03 00:00:00
  00d34668025906d55ae2e529615f530a,2006-03-09 00:00:00
  00d34668025906d55ae2e529615f530a,2006-03-09 00:00:00
  0101d3286dfbd58642a7527ecbddb92e,2007-10-13 00:00:00
  0101d3286dfbd58642a7527ecbddb92e,2007-10-27 00:00:00
  0103bd73af66e5a44f7867c0bb2203cc,2001-02-01 00:00:00
  0103bd73af66e5a44f7867c0bb2203cc,2008-01-20 00:00:00
  '''
  df = pandas.read_csv(StringIO.StringIO(txt))
  df = df.sort('DATE')
  df.DATE = pandas.to_datetime(df.DATE)
  df['X_DATE'] = df['DATE'].shift(180, freq=pandas.datetools.Day)
  </code></pre>
  
  <p>This code generates a type error.  For reference I am using:</p>
  
  <p>Python 2.7.4
  Pandas '0.12.0.dev-6e7c4d6'
  Numpy '1.7.1'</p>