diff --git a/tests/test_algorithm_gen.py b/tests/test_algorithm_gen.py
index 6de78158..a2a395c0 100644
--- a/tests/test_algorithm_gen.py
+++ b/tests/test_algorithm_gen.py
@@ -18,8 +18,9 @@ from unittest import TestCase
 from nose.tools import timed
 
 from datetime import datetime
-import pytz
 
+import pytz
+import zipline.finance.trading as trading
 from zipline.algorithm import TradingAlgorithm
 from zipline.finance import slippage
 from zipline.utils import factory
@@ -78,6 +79,35 @@ class AlgorithmGeneratorTestCase(TestCase):
     def tearDown(self):
         teardown_logger(self)
 
+    def test_lse_algorithm(self):
+
+        lse = trading.TradingEnvironment(
+            bm_symbol='^FTSE',
+            exchange_tz='Europe/London'
+        )
+
+        with lse:
+
+            sim_params = factory.create_simulation_parameters(
+                start=datetime(2012, 5, 1, tzinfo=pytz.utc),
+                end=datetime(2012, 6, 30, tzinfo=pytz.utc)
+            )
+            algo = TestAlgo(self, sim_params=sim_params)
+            trade_source = factory.create_daily_trade_source(
+                [8229],
+                200,
+                sim_params
+            )
+            algo.set_sources([trade_source])
+
+            gen = algo.get_generator()
+            results = list(gen)
+            self.assertEqual(len(results), 42)
+            # May 7, 2012 was an LSE holiday, confirm the 4th trading
+            # day was May 8.
+            self.assertEqual(results[4]['daily_perf']['period_open'],
+                             datetime(2012, 5, 8, 8, 30, tzinfo=pytz.utc))
+
     @timed(DEFAULT_TIMEOUT)
     def test_generator_dates(self):
         """
diff --git a/tests/test_risk_compare_batch_iterative.py b/tests/test_risk_compare_batch_iterative.py
index d98fbe94..4d5a7828 100644
--- a/tests/test_risk_compare_batch_iterative.py
+++ b/tests/test_risk_compare_batch_iterative.py
@@ -22,7 +22,6 @@ import numpy as np
 
 import zipline.finance.risk as risk
 
-from zipline.finance.trading import TradingEnvironment
 import zipline.finance.trading as trading
 from test_risk import RETURNS
 
@@ -44,8 +43,6 @@ class RiskCompareIterativeToBatch(unittest.TestCase):
         self.end_date = datetime.datetime(
             year=2006, month=12, day=31, tzinfo=pytz.utc)
 
-        # setup the default trading environment
-        trading.environment = TradingEnvironment()
         self.oneday = datetime.timedelta(days=1)
 
     def test_risk_metrics_returns(self):
@@ -55,7 +52,7 @@ class RiskCompareIterativeToBatch(unittest.TestCase):
 
         cur_returns = []
         for i, ret in enumerate(RETURNS):
-            todays_return_obj = risk.DailyReturn(
+            todays_return_obj = trading.DailyReturn(
                 todays_date,
                 ret
             )
diff --git a/tests/test_tradingcalendar.py b/tests/test_tradingcalendar.py
index 6c29c077..a5416413 100644
--- a/tests/test_tradingcalendar.py
+++ b/tests/test_tradingcalendar.py
@@ -15,13 +15,20 @@
 
 from unittest import TestCase
 from zipline.utils import tradingcalendar
+from zipline.utils import tradingcalendar_lse
 import pytz
 import datetime
 from zipline.finance.trading import TradingEnvironment
+from pandas import DatetimeIndex
+from delorean import Delorean
 
 
 class TestTradingCalendar(TestCase):
 
+    def setUp(self):
+        today = Delorean().truncate('day')
+        self.end = DatetimeIndex([today.datetime])
+
     def test_calendar_vs_environment(self):
         """
         test_calendar_vs_environment checks whether the
@@ -41,6 +48,44 @@ class TestTradingCalendar(TestCase):
             "{diff} should be empty".format(diff=diff)
         )
 
+        diff2 = tradingcalendar.trading_days - env_days
+        # depending on the time of day, data for the current day
+        # may not be available from yahoo, so don't include end
+        # of the tradingcalendar
+        diff2 = diff2 - self.end
+        self.assertEqual(
+            len(diff2),
+            0,
+            "{diff} should be empty".format(diff=diff2)
+        )
+
+    def test_lse_calendar_vs_environment(self):
+        env = TradingEnvironment(
+            bm_symbol='^FTSE',
+            exchange_tz='Europe/London'
+        )
+
+        env_start_index = \
+            env.trading_days.searchsorted(tradingcalendar_lse.start)
+        env_days = env.trading_days[env_start_index:]
+        diff = env_days - tradingcalendar_lse.trading_days
+        self.assertEqual(
+            len(diff),
+            0,
+            "{diff} should be empty".format(diff=diff)
+        )
+
+        diff2 = tradingcalendar_lse.trading_days - env_days
+        # depending on the time of day, data for the current day
+        # may not be available from yahoo, so don't include end
+        # of the tradingcalendar
+        diff2 = diff2 - self.end
+        self.assertEqual(
+            len(diff2),
+            0,
+            "{diff} should be empty".format(diff=diff2)
+        )
+
     def test_newyears(self):
         """
         Check whether tradingcalendar contains certain dates.
diff --git a/zipline/data/benchmarks.py b/zipline/data/benchmarks.py
index 919b0f92..8bdaa20a 100644
--- a/zipline/data/benchmarks.py
+++ b/zipline/data/benchmarks.py
@@ -30,7 +30,6 @@ from loader_utils import (
 
 from loader_utils import Mapping
 
-from zipline.finance.risk import DailyReturn
 
 _BENCHMARK_MAPPING = {
     # Need to add 'symbol'
@@ -95,6 +94,7 @@ def get_benchmark_data(symbol):
 
 
 def get_benchmark_returns(symbol):
+    from zipline.finance.trading import DailyReturn
 
     benchmark_returns = []
 
diff --git a/zipline/data/loader.py b/zipline/data/loader.py
index 4ebe4a23..21586da2 100644
--- a/zipline/data/loader.py
+++ b/zipline/data/loader.py
@@ -24,7 +24,6 @@ from treasuries import get_treasury_data
 from benchmarks import get_benchmark_returns
 
 from zipline.utils.date_utils import tuple_to_date
-import zipline.finance.risk as risk
 from operator import attrgetter
 
 
@@ -92,6 +91,8 @@ def get_benchmark_filename(symbol):
 
 
 def load_market_data(bm_symbol='^GSPC'):
+    from zipline.finance.trading import DailyReturn
+
     try:
         fp_bm = get_datafile(get_benchmark_filename(bm_symbol), "rb")
     except IOError:
@@ -107,7 +108,7 @@ Fetching data from Yahoo Finance.
     for packed_date, returns in bm_list:
         event_dt = tuple_to_date(packed_date)
 
-        daily_return = risk.DailyReturn(date=event_dt, returns=returns)
+        daily_return = DailyReturn(date=event_dt, returns=returns)
         bm_returns.append(daily_return)
 
     fp_bm.close()
diff --git a/zipline/finance/performance.py b/zipline/finance/performance.py
index 10306b68..5c84e8af 100644
--- a/zipline/finance/performance.py
+++ b/zipline/finance/performance.py
@@ -278,7 +278,10 @@ class PerformanceTracker(object):
     def handle_market_close(self):
         # add the return results from today to the list of DailyReturn objects.
         todays_date = self.market_close.replace(hour=0, minute=0, second=0)
-        todays_return_obj = risk.DailyReturn(
+        self.cumulative_performance.update_dividends(todays_date)
+        self.todays_performance.update_dividends(todays_date)
+
+        todays_return_obj = trading.DailyReturn(
             todays_date,
             self.todays_performance.returns
         )
diff --git a/zipline/finance/risk.py b/zipline/finance/risk.py
index 1a6a46e0..b8fb2806 100644
--- a/zipline/finance/risk.py
+++ b/zipline/finance/risk.py
@@ -94,24 +94,6 @@ def advance_by_months(dt, jump_in_months):
     return dt.replace(year=dt.year + years, month=month)
 
 
-class DailyReturn(object):
-
-    def __init__(self, date, returns):
-
-        assert isinstance(date, datetime.datetime)
-        self.date = date.replace(hour=0, minute=0, second=0)
-        self.returns = returns
-
-    def to_dict(self):
-        return {
-            'dt': self.date,
-            'returns': self.returns
-        }
-
-    def __repr__(self):
-        return str(self.date) + " - " + str(self.returns)
-
-
 class RiskMetricsBase(object):
     def __init__(self, start_date, end_date, returns):
 
diff --git a/zipline/finance/trading.py b/zipline/finance/trading.py
index 7f1b92af..d01adca1 100644
--- a/zipline/finance/trading.py
+++ b/zipline/finance/trading.py
@@ -32,6 +32,43 @@ from zipline.finance.commission import PerShare
 
 log = logbook.Logger('Transaction Simulator')
 
+
+# The financial simulations in zipline depend on information
+# about the benchmark index and the risk free rates of return.
+# The benchmark index defines the benchmark returns used in
+# the calculation of performance metrics such as alpha/beta. Many
+# components, including risk, performance, transforms, and
+# batch_transforms, need access to a calendar of trading days and
+# market hours. The TradingEnvironment maintains two time keeping
+# facilities:
+#   - a DatetimeIndex of trading days for calendar calculations
+#   - a timezone name, which should be local to the exchange
+#   hosting the benchmark index. All dates are normalized to UTC
+#   for serialization and storage, and the timezone is used to
+#   ensure proper rollover through daylight savings and so on.
+#
+# This module maintains a global variable, environment, which is
+# subsequently referenced directly by zipline financial
+# components. To set the environment, you can set the property on
+# the module directly:
+#       import zipline.finance.trading as trading
+#       trading.environment = TradingEnvironment()
+#
+# or if you want to switch the environment for a limited context
+# you can use a TradingEnvironment in a with clause:
+#       lse = TradingEnvironment(bm_index="^FTSE", exchange_tz="Europe/London")
+#       with lse:
+#           # the code here will have lse as the global trading.environment
+#           algo.run(start, end)
+#
+# User code will not normally need to use TradingEnvironment
+# directly. If you are extending zipline's core financial
+# compponents and need to use the environment, you must import the module
+# NOT the variable. If you import the module, you will get a
+# reference to the environment at import time, which will prevent
+# your code from responding to user code that changes the global
+# state.
+
 environment = None
 
 
@@ -69,7 +106,7 @@ class TradingEnvironment(object):
         bm_symbol='^GSPC',
         exchange_tz="US/Eastern"
     ):
-
+        self.prev_environment = self
         self.trading_day_map = OrderedDict()
         self.bm_symbol = bm_symbol
         if not load:
@@ -90,6 +127,21 @@ class TradingEnvironment(object):
         self.first_trading_day = next(self.trading_day_map.iterkeys())
         self.last_trading_day = next(reversed(self.trading_day_map))
 
+    def __enter__(self, *args, **kwargs):
+        global environment
+        self.prev_environment = environment
+        environment = self
+        # return value here is associated with "as such_and_such" on the
+        # with clause.
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        global environment
+        environment = self.prev_environment
+        # signal that any exceptions need to be propagated up the
+        # stack.
+        return False
+
     def normalize_date(self, test_date):
         return datetime.datetime(
             year=test_date.year,
@@ -165,7 +217,7 @@ Last successful date: %s" % self.market_open)
         )
         # create a new Delorean with the next_open naive date and
         # the correct timezone for the exchange.
-        open_delorean = Delorean(next_open, "US/Eastern")
+        open_delorean = Delorean(next_open, self.exchange_tz)
         open_utc = open_delorean.shift("UTC").datetime
 
         market_open = open_utc
@@ -202,10 +254,9 @@ class SimulationParameters(object):
     def __init__(self, period_start, period_end,
                  capital_base=10e3):
 
-        # raise and exception if the global environment is not
-        # set.
         global environment
         if not environment:
+            # This is the global environment for trading simulation.
             environment = TradingEnvironment()
 
         self.period_start = period_start
@@ -287,3 +338,21 @@ class SimulationParameters(object):
             {'first_open': self.first_open,
              'last_close': self.last_close
              })
+
+
+class DailyReturn(object):
+
+    def __init__(self, date, returns):
+
+        assert isinstance(date, datetime.datetime)
+        self.date = date.replace(hour=0, minute=0, second=0)
+        self.returns = returns
+
+    def to_dict(self):
+        return {
+            'dt': self.date,
+            'returns': self.returns
+        }
+
+    def __repr__(self):
+        return str(self.date) + " - " + str(self.returns)
diff --git a/zipline/utils/factory.py b/zipline/utils/factory.py
index 066ba414..34c8b064 100644
--- a/zipline/utils/factory.py
+++ b/zipline/utils/factory.py
@@ -26,13 +26,12 @@ from pandas.io.data import DataReader
 import numpy as np
 from datetime import datetime, timedelta
 
-import zipline.finance.risk as risk
 from zipline.protocol import Event, DATASOURCE_TYPE
 from zipline.sources import (SpecificEquityTrades,
                              DataFrameSource,
                              DataPanelSource)
 from zipline.gens.utils import create_trade
-from zipline.finance.trading import SimulationParameters, TradingEnvironment
+from zipline.finance.trading import SimulationParameters
 import zipline.finance.trading as trading
 
 
@@ -40,7 +39,6 @@ def create_simulation_parameters(year=2006, start=None, end=None,
                                  capital_base=float("1.0e5")
                                  ):
     """Construct a complete environment with reasonable defaults"""
-    trading.environment = TradingEnvironment()
     if start is None:
         start = datetime(year, 1, 1, tzinfo=pytz.utc)
     if end is None:
@@ -56,34 +54,33 @@ def create_simulation_parameters(year=2006, start=None, end=None,
 
 
 def create_random_simulation_parameters():
-        trading.environment = TradingEnvironment()
-        treasury_curves = trading.environment.treasury_curves
+    treasury_curves = trading.environment.treasury_curves
 
-        for n in range(100):
+    for n in range(100):
 
-            random_index = random.randint(
-                0,
-                len(treasury_curves)
-            )
+        random_index = random.randint(
+            0,
+            len(treasury_curves)
+        )
 
-            start_dt = treasury_curves.keys()[random_index]
-            end_dt = start_dt + timedelta(days=365)
+        start_dt = treasury_curves.keys()[random_index]
+        end_dt = start_dt + timedelta(days=365)
 
-            now = datetime.utcnow().replace(tzinfo=pytz.utc)
+        now = datetime.utcnow().replace(tzinfo=pytz.utc)
 
-            if end_dt <= now:
-                break
+        if end_dt <= now:
+            break
 
-        assert end_dt <= now, """
+    assert end_dt <= now, """
 failed to find a suitable daterange after 100 attempts. please double
 check treasury and benchmark data in findb, and re-run the test."""
 
-        sim_params = SimulationParameters(
-            period_start=start_dt,
-            period_end=end_dt
-        )
+    sim_params = SimulationParameters(
+        period_start=start_dt,
+        period_end=end_dt
+    )
 
-        return sim_params, start_dt, end_dt
+    return sim_params, start_dt, end_dt
 
 
 def get_next_trading_dt(current, interval):
@@ -158,7 +155,7 @@ def create_returns(daycount, sim_params):
     for day in range(daycount):
         current = current + one_day
         if trading.environment.is_trading_day(current):
-            r = risk.DailyReturn(current, random.random())
+            r = trading.DailyReturn(current, random.random())
             test_range.append(r)
 
     return test_range
@@ -170,7 +167,7 @@ def create_returns_from_range(sim_params):
     one_day = timedelta(days=1)
     test_range = []
     while current <= end:
-        r = risk.DailyReturn(current, random.random())
+        r = trading.DailyReturn(current, random.random())
         test_range.append(r)
         current = get_next_trading_dt(current, one_day)
 
@@ -187,7 +184,7 @@ def create_returns_from_list(returns, sim_params):
         current = get_next_trading_dt(current, one_day)
 
     for return_val in returns:
-        r = risk.DailyReturn(current, return_val)
+        r = trading.DailyReturn(current, return_val)
         test_range.append(r)
         current = get_next_trading_dt(current, one_day)
 
diff --git a/zipline/utils/tradingcalendar.py b/zipline/utils/tradingcalendar.py
index 08d63fda..36d35398 100644
--- a/zipline/utils/tradingcalendar.py
+++ b/zipline/utils/tradingcalendar.py
@@ -227,6 +227,8 @@ def get_non_trading_days(start, end):
     # http://www.nyse.com/pdfs/closings.pdf
     #
     # National Days of Mourning
+    # - President Richard Nixon
+    non_trading_days.append(datetime(1994, 4, 27, tzinfo=pytz.utc))
     # - President Ronald W. Reagan - June 11, 2004
     non_trading_days.append(datetime(2004, 6, 11, tzinfo=pytz.utc))
     # - President Gerald R. Ford - Jan 2, 2007
diff --git a/zipline/utils/tradingcalendar_lse.py b/zipline/utils/tradingcalendar_lse.py
new file mode 100644
index 00000000..60f2d72b
--- /dev/null
+++ b/zipline/utils/tradingcalendar_lse.py
@@ -0,0 +1,187 @@
+#
+# Copyright 2012 Quantopian, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+# References:
+# http://www.londonstockexchange.com
+# /about-the-exchange/company-overview/business-days/business-days.htm
+# http://en.wikipedia.org/wiki/Bank_holiday
+# http://www.adviceguide.org.uk/england/work_e/work_time_off_work_e/
+# bank_and_public_holidays.htm
+
+import pytz
+
+import pandas as pd
+
+from datetime import datetime
+from dateutil import rrule
+from zipline.utils.date_utils import utcnow
+
+start = datetime(2002, 1, 1, tzinfo=pytz.utc)
+end = utcnow()
+
+non_trading_rules = []
+# Weekends
+weekends = rrule.rrule(
+    rrule.YEARLY,
+    byweekday=(rrule.SA, rrule.SU),
+    cache=True,
+    dtstart=start,
+    until=end
+)
+non_trading_rules.append(weekends)
+# New Year's Day
+new_year = rrule.rrule(
+    rrule.MONTHLY,
+    byyearday=1,
+    cache=True,
+    dtstart=start,
+    until=end
+)
+# If new years day is on Saturday then Monday 3rd is a holiday
+# If new years day is on Sunday then Monday 2nd is a holiday
+weekend_new_year = rrule.rrule(
+    rrule.MONTHLY,
+    bymonth=1,
+    bymonthday=[2, 3],
+    byweekday=(rrule.MO),
+    cache=True,
+    dtstart=start,
+    until=end
+)
+non_trading_rules.append(new_year)
+non_trading_rules.append(weekend_new_year)
+# Good Friday
+good_friday = rrule.rrule(
+    rrule.DAILY,
+    byeaster=-2,
+    cache=True,
+    dtstart=start,
+    until=end
+)
+non_trading_rules.append(good_friday)
+# Easter Monday
+easter_monday = rrule.rrule(
+    rrule.DAILY,
+    byeaster=1,
+    cache=True,
+    dtstart=start,
+    until=end
+)
+non_trading_rules.append(easter_monday)
+# Early May Bank Holiday (1st Monday in May)
+may_bank = rrule.rrule(
+    rrule.MONTHLY,
+    bymonth=5,
+    byweekday=(rrule.MO(1)),
+    cache=True,
+    dtstart=start,
+    until=end
+)
+non_trading_rules.append(may_bank)
+# Spring Bank Holiday (Last Monday in May)
+spring_bank = rrule.rrule(
+    rrule.MONTHLY,
+    bymonth=5,
+    byweekday=(rrule.MO(-1)),
+    cache=True,
+    dtstart=datetime(2003, 1, 1, tzinfo=pytz.utc),
+    until=end
+)
+non_trading_rules.append(spring_bank)
+# Summer Bank Holiday (Last Monday in August)
+summer_bank = rrule.rrule(
+    rrule.MONTHLY,
+    bymonth=8,
+    byweekday=(rrule.MO(-1)),
+    cache=True,
+    dtstart=start,
+    until=end
+)
+non_trading_rules.append(summer_bank)
+# Christmas Day
+christmas = rrule.rrule(
+    rrule.MONTHLY,
+    bymonth=12,
+    bymonthday=25,
+    cache=True,
+    dtstart=start,
+    until=end
+)
+# If christmas day is Saturday Monday 27th is a holiday
+# If christmas day is sunday the Tuesday 27th is a holiday
+weekend_christmas = rrule.rrule(
+    rrule.MONTHLY,
+    bymonth=12,
+    bymonthday=27,
+    byweekday=(rrule.MO, rrule.TU),
+    cache=True,
+    dtstart=start,
+    until=end
+)
+
+non_trading_rules.append(christmas)
+non_trading_rules.append(weekend_christmas)
+# Boxing Day
+boxing_day = rrule.rrule(
+    rrule.MONTHLY,
+    bymonth=12,
+    bymonthday=26,
+    cache=True,
+    dtstart=start,
+    until=end
+)
+# If boxing day is saturday then Monday 28th is a holiday
+# If boxing day is sunday then Tuesday 28th is a holiday
+weekend_boxing_day = rrule.rrule(
+    rrule.MONTHLY,
+    bymonth=12,
+    bymonthday=28,
+    byweekday=(rrule.MO, rrule.TU),
+    cache=True,
+    dtstart=start,
+    until=end
+)
+
+non_trading_rules.append(boxing_day)
+non_trading_rules.append(weekend_boxing_day)
+
+non_trading_ruleset = rrule.rruleset()
+
+# In 2002 May bank holiday was moved to 4th June to follow the Queens
+# Golden Jubilee
+non_trading_ruleset.exdate(datetime(2002, 9, 27, tzinfo=pytz.utc))
+non_trading_ruleset.rdate(datetime(2002, 6, 3, tzinfo=pytz.utc))
+non_trading_ruleset.rdate(datetime(2002, 6, 4, tzinfo=pytz.utc))
+# TODO: not sure why Feb 18 2008 is not available in the yahoo data
+non_trading_ruleset.rdate(datetime(2008, 2, 18, tzinfo=pytz.utc))
+# In 2011 The Friday before Mayday was the Royal Wedding
+non_trading_ruleset.rdate(datetime(2011, 4, 29, tzinfo=pytz.utc))
+# In 2012 May bank holiday was moved to 4th June to preceed the Queens
+# Diamond Jubilee
+non_trading_ruleset.exdate(datetime(2012, 5, 28, tzinfo=pytz.utc))
+non_trading_ruleset.rdate(datetime(2012, 6, 4, tzinfo=pytz.utc))
+non_trading_ruleset.rdate(datetime(2012, 6, 5, tzinfo=pytz.utc))
+
+for rule in non_trading_rules:
+    non_trading_ruleset.rrule(rule)
+
+non_trading_days = non_trading_ruleset.between(start, end, inc=True)
+non_trading_day_index = pd.DatetimeIndex(sorted(non_trading_days))
+
+business_days = pd.DatetimeIndex(start=start, end=end,
+                                 freq=pd.datetools.BDay())
+
+trading_days = business_days - non_trading_day_index