title: Pandas CustomBusinessDay offset subtract from datetime
body: <p>I have the following code for a custom offset class of market holidays. For some reason though I can not subtract an offset. Addition works fine. Can someone spot the error? </p>
  
  <pre><code>from pandas.tseries.offsets import CustomBusinessDay
  from pandas.tseries.holiday import get_calendar, HolidayCalendarFactory, GoodFriday
  
  
  class Holiday_offset:
  '''
      creates an offset for market holidays so that the object that is returned
      may be added or subtracted to a datetime object to move 1 day forward or 
      back account for holidays and business days.
  '''
  def __init__(self):
      self.cal = get_calendar('USFederalHolidayCalendar')  # Create calendar instance
      self.cal.rules.pop(7)                                # Remove Veteran's Day rule
      self.cal.rules.pop(6)                                # Remove Columbus Day rule
      self.tradingCal = HolidayCalendarFactory('TradingCalendar', self.cal, GoodFriday)
  
      #new instance of class return as an offset object 
      self.offset = CustomBusinessDay(calendar=self.tradingCal())
  def __add__(self,other):
      return self.offset + other
  def __radd__(self,other):
      return self.offset + other
  def __iadd__(self,other):
      return self.offset + other
  def __sub__(self,other):
      return self.offset - other
  def __rsub__(self,other):
      return self.offset - other
  def __isub__(self,other):
      return self.offset - other
  def __index__(self,other):
  
  
  
  TypeError: Cannot subtract datetime from offset
  </code></pre>