ENH: allows users to specify the cutoff time for data query in blaze

loaders

This allows people to set their cutoff time to the time they will
actually execute 'before_trading_start'. Currently this is just passed
to the constructor of the loader; however, I would like to make this
managed by the algorithm simulation runner. This would help keep all of
the loaders in sync and lock 'before_trading_start's execution to the
time the data is queried for.

tests/utils/test_preprocess.py

@@ -7,10 +7,12 @@ from unittest import TestCase
 
 from nose_parameterized import parameterized
 from numpy import arange, dtype
+import pytz
 from six import PY3
 
 from zipline.utils.preprocess import call, preprocess
 from zipline.utils.input_validation import (
+    ensure_timezone,
     expect_element,
     expect_dtypes,
     expect_types,
@@ -317,3 +319,31 @@ class PreprocessTestCase(TestCase):
             "or 'float64' for argument 'a', but got 'uint32' instead."
         ).format(qualname=qualname(foo))
         self.assertEqual(e.exception.args[0], expected_message)
+
+    def test_ensure_timezone(self):
+        @preprocess(tz=ensure_timezone)
+        def f(tz):
+            return tz
+
+        valid = {
+            'utc',
+            'EST',
+            'US/Eastern',
+        }
+        invalid = {
+            # unfortunatly, these are not actually timezones (yet)
+            'ayy',
+            'lmao',
+        }
+
+        # test coercing from string
+        for tz in valid:
+            self.assertEqual(f(tz), pytz.timezone(tz))
+
+        # test pass through of tzinfo objects
+        for tz in map(pytz.timezone, valid):
+            self.assertEqual(f(tz), tz)
+
+        # test invalid timezone strings
+        for tz in invalid:
+            self.assertRaises(pytz.UnknownTimeZoneError, f, tz)