code: def balance(self, date=None):
          
  
          qs = self._entries()
          if date:
              qs = qs.filter(transaction__t_stamp__lt=date)
          r = qs.aggregate(b=Sum('amount'))
          b = r['b']
  
          flip = self._DEBIT_IN_DB()
          if self._positive_credit():
              flip *= -1
  
          if b is None:
              b = Decimal("0.00")
          b *= flip
  
          #print "returning balance %s for %s" % (b, self)
          return b
docs: returns the account balance as of 'date' (datetime stamp) or now().