diff --git a/stix2/core.py b/stix2/core.py
index 5d5ae3b..9594e49 100644
--- a/stix2/core.py
+++ b/stix2/core.py
@@ -29,10 +29,9 @@ def parse(data, allow_custom=False, version=None):
         allow_custom (bool): Whether to allow custom properties as well unknown
             custom objects. Note that unknown custom objects cannot be parsed
             into STIX objects, and will be returned as is. Default: False.
-        version (str): Only used for bundles.  If the spec_version property is
-            missing, it is ambiguous what spec should be used to parse the
-            bundle.  In this case, this version parameter gives the spec
-            version to use.
+        version (str): If present, it forces the parser to use the version
+            provided. Otherwise, the library will make the best effort based
+            on checking the "spec_version" property.
 
     Returns:
         An instantiated Python STIX object.
@@ -59,30 +58,29 @@ def parse(data, allow_custom=False, version=None):
 def dict_to_stix2(stix_dict, allow_custom=False, version=None):
     """convert dictionary to full python-stix2 object
 
-        Args:
-            stix_dict (dict): a python dictionary of a STIX object
-                that (presumably) is semantically correct to be parsed
-                into a full python-stix2 obj
-            allow_custom (bool): Whether to allow custom properties as well
-                unknown custom objects. Note that unknown custom objects cannot
-                be parsed into STIX objects, and will be returned as is.
-                Default: False.
-            version: Only used for bundles.  If the spec_version property is
-                missing, it is ambiguous what spec should be used to parse the
-                bundle.  In this case, this version parameter gives the spec
-                version to use.
+    Args:
+        stix_dict (dict): a python dictionary of a STIX object
+            that (presumably) is semantically correct to be parsed
+            into a full python-stix2 obj
+        allow_custom (bool): Whether to allow custom properties as well
+            unknown custom objects. Note that unknown custom objects cannot
+            be parsed into STIX objects, and will be returned as is.
+            Default: False.
+        version (str): If present, it forces the parser to use the version
+            provided. Otherwise, the library will make the best effort based
+            on checking the "spec_version" property.
 
-        Returns:
-            An instantiated Python STIX object
+    Returns:
+        An instantiated Python STIX object
 
-        Warnings:
-            'allow_custom=True' will allow for the return of any supplied STIX
-            dict(s) that cannot be found to map to any known STIX object types
-            (both STIX2 domain objects or defined custom STIX2 objects); NO
-            validation is done. This is done to allow the processing of
-            possibly unknown custom STIX objects (example scenario: I need to
-            query a third-party TAXII endpoint that could provide custom STIX
-            objects that I don't know about ahead of time)
+    Warnings:
+        'allow_custom=True' will allow for the return of any supplied STIX
+        dict(s) that cannot be found to map to any known STIX object types
+        (both STIX2 domain objects or defined custom STIX2 objects); NO
+        validation is done. This is done to allow the processing of
+        possibly unknown custom STIX objects (example scenario: I need to
+        query a third-party TAXII endpoint that could provide custom STIX
+        objects that I don't know about ahead of time)
 
     """
     if 'type' not in stix_dict:
@@ -130,11 +128,12 @@ def parse_observable(data, _valid_refs=None, allow_custom=False, version=None):
             object being parsed. Use empty list if no valid refs are present.
         allow_custom (bool): Whether to allow custom properties or not.
             Default: False.
-        version (str): If the spec version is missing, the latest supported
-            stix2 version will be used to parse the observable object.
+        version (str): If present, it forces the parser to use the version
+            provided. Otherwise, the library will use the latest version.
 
     Returns:
         An instantiated Python STIX Cyber Observable object.
+
     """
     obj = _get_dict(data)
     # get deep copy since we are going modify the dict and might
@@ -187,6 +186,7 @@ def _register_object(new_type, version=None):
         new_type (class): A class to register in the Object map.
         version (str): Which STIX2 version to use. (e.g. "2.0", "2.1"). If
             None, use latest version.
+
     """
     if version:
         v = 'v' + version.replace('.', '')
@@ -205,6 +205,7 @@ def _register_marking(new_marking, version=None):
         new_marking (class): A class to register in the Marking map.
         version (str): Which STIX2 version to use. (e.g. "2.0", "2.1"). If
             None, use latest version.
+
     """
     if version:
         v = 'v' + version.replace('.', '')
@@ -223,6 +224,7 @@ def _register_observable(new_observable, version=None):
         new_observable (class): A class to register in the Observables map.
         version (str): Which STIX2 version to use. (e.g. "2.0", "2.1"). If
             None, use latest version.
+
     """
     if version:
         v = 'v' + version.replace('.', '')
@@ -243,6 +245,7 @@ def _register_observable_extension(observable, new_extension, version=None):
             Extensions map.
         version (str): Which STIX2 version to use. (e.g. "2.0", "2.1"). If
             None, use latest version.
+
     """
     if version:
         v = 'v' + version.replace('.', '')
diff --git a/stix2/datastore/memory.py b/stix2/datastore/memory.py
index 819c0bf..7d5fb1c 100644
--- a/stix2/datastore/memory.py
+++ b/stix2/datastore/memory.py
@@ -21,7 +21,11 @@ def _add(store, stix_data, allow_custom=True, version=None):
     Recursive function, breaks down STIX Bundles and lists.
 
     Args:
+        store: A MemoryStore, MemorySink or MemorySource object.
         stix_data (list OR dict OR STIX object): STIX objects to be added
+        allow_custom (bool): Whether to allow custom properties as well unknown
+            custom objects. Note that unknown custom objects cannot be parsed
+            into STIX objects, and will be returned as is. Default: False.
         version (str): Which STIX2 version to lock the parser to. (e.g. "2.0",
             "2.1"). If None, the library makes the best effort to figure
             out the spec representation of the object.
@@ -93,8 +97,10 @@ class _ObjectFamily(object):
             self.latest_version = obj
 
     def __str__(self):
-        return "<<{}; latest={}>>".format(self.all_versions,
-                                          self.latest_version.modified)
+        return "<<{}; latest={}>>".format(
+            self.all_versions,
+            self.latest_version.modified,
+        )
 
     def __repr__(self):
         return str(self)
@@ -278,8 +284,8 @@ class MemorySource(DataSource):
             all_filters = list(
                 itertools.chain(
                     _composite_filters or [],
-                    self.filters
-                )
+                    self.filters,
+                ),
             )
 
             stix_obj = next(apply_common_filters([stix_obj], all_filters), None)
@@ -317,12 +323,12 @@ class MemorySource(DataSource):
             all_filters = list(
                 itertools.chain(
                     _composite_filters or [],
-                    self.filters
-                )
+                    self.filters,
+                ),
             )
 
             results.extend(
-                apply_common_filters(stix_objs_to_filter, all_filters)
+                apply_common_filters(stix_objs_to_filter, all_filters),
             )
 
         return results
diff --git a/stix2/test/v20/test_datastore_memory.py b/stix2/test/v20/test_datastore_memory.py
index ec27f3f..ae3f754 100644
--- a/stix2/test/v20/test_datastore_memory.py
+++ b/stix2/test/v20/test_datastore_memory.py
@@ -5,10 +5,10 @@ import pytest
 
 from stix2 import Filter, MemorySource, MemoryStore, properties
 from stix2.datastore import make_id
+from stix2.utils import parse_into_datetime
 from stix2.v20 import (
     Bundle, Campaign, CustomObject, Identity, Indicator, Malware, Relationship,
 )
-from stix2.utils import parse_into_datetime
 
 from .constants import (
     CAMPAIGN_ID, CAMPAIGN_KWARGS, IDENTITY_ID, IDENTITY_KWARGS, INDICATOR_ID,
diff --git a/stix2/test/v20/test_datastore_taxii.py b/stix2/test/v20/test_datastore_taxii.py
index a9765cb..9ac3a62 100644
--- a/stix2/test/v20/test_datastore_taxii.py
+++ b/stix2/test/v20/test_datastore_taxii.py
@@ -18,7 +18,7 @@ class MockTAXIICollectionEndpoint(Collection):
 
     def __init__(self, url, collection_info):
         super(MockTAXIICollectionEndpoint, self).__init__(
-            url, collection_info=collection_info
+            url, collection_info=collection_info,
         )
         self.objects = []
 
@@ -80,7 +80,7 @@ def collection(stix_objs1):
             "media_types": [
                 "application/vnd.oasis.stix+json; version=2.0",
             ],
-        }
+        },
     )
 
     mock.objects.extend(stix_objs1)
@@ -99,7 +99,7 @@ def collection_no_rw_access(stix_objs1):
             "media_types": [
                 "application/vnd.oasis.stix+json; version=2.0",
             ],
-        }
+        },
     )
 
     mock.objects.extend(stix_objs1)
diff --git a/stix2/test/v21/test_datastore_composite.py b/stix2/test/v21/test_datastore_composite.py
index 39c7c48..76119c3 100644
--- a/stix2/test/v21/test_datastore_composite.py
+++ b/stix2/test/v21/test_datastore_composite.py
@@ -2,7 +2,9 @@ import pytest
 
 from stix2.datastore import CompositeDataSource, make_id
 from stix2.datastore.filters import Filter
-from stix2.datastore.memory import MemorySink, MemorySource
+from stix2.datastore.memory import MemorySink, MemorySource, MemoryStore
+from stix2.utils import parse_into_datetime
+from stix2.v21.common import TLP_GREEN
 
 
 def test_add_remove_composite_datasource():
@@ -47,14 +49,14 @@ def test_composite_datasource_operations(stix_objs1, stix_objs2):
     indicators = cds1.all_versions("indicator--00000000-0000-4000-8000-000000000001")
 
     # In STIX_OBJS2 changed the 'modified' property to a later time...
-    assert len(indicators) == 2
+    assert len(indicators) == 3
 
     cds1.add_data_sources([cds2])
 
     indicator = cds1.get("indicator--00000000-0000-4000-8000-000000000001")
 
     assert indicator["id"] == "indicator--00000000-0000-4000-8000-000000000001"
-    assert indicator["modified"] == "2017-01-31T13:49:53.935Z"
+    assert indicator["modified"] == parse_into_datetime("2017-01-31T13:49:53.935Z")
     assert indicator["type"] == "indicator"
 
     query1 = [
@@ -71,20 +73,80 @@ def test_composite_datasource_operations(stix_objs1, stix_objs2):
 
     # STIX_OBJS2 has indicator with later time, one with different id, one with
     # original time in STIX_OBJS1
-    assert len(results) == 3
+    assert len(results) == 4
 
     indicator = cds1.get("indicator--00000000-0000-4000-8000-000000000001")
 
     assert indicator["id"] == "indicator--00000000-0000-4000-8000-000000000001"
-    assert indicator["modified"] == "2017-01-31T13:49:53.935Z"
+    assert indicator["modified"] == parse_into_datetime("2017-01-31T13:49:53.935Z")
     assert indicator["type"] == "indicator"
 
-    # There is only one indicator with different ID. Since we use the same data
-    # when deduplicated, only two indicators (one with different modified).
     results = cds1.all_versions("indicator--00000000-0000-4000-8000-000000000001")
-    assert len(results) == 2
+    assert len(results) == 3
 
     # Since we have filters already associated with our CompositeSource providing
     # nothing returns the same as cds1.query(query1) (the associated query is query2)
     results = cds1.query([])
-    assert len(results) == 3
+    assert len(results) == 4
+
+
+def test_source_markings():
+    msrc = MemorySource(TLP_GREEN)
+
+    assert msrc.get(TLP_GREEN.id) == TLP_GREEN
+    assert msrc.all_versions(TLP_GREEN.id) == [TLP_GREEN]
+    assert msrc.query(Filter("id", "=", TLP_GREEN.id)) == [TLP_GREEN]
+
+
+def test_sink_markings():
+    # just make sure there is no crash
+    msink = MemorySink(TLP_GREEN)
+    msink.add(TLP_GREEN)
+
+
+def test_store_markings():
+    mstore = MemoryStore(TLP_GREEN)
+
+    assert mstore.get(TLP_GREEN.id) == TLP_GREEN
+    assert mstore.all_versions(TLP_GREEN.id) == [TLP_GREEN]
+    assert mstore.query(Filter("id", "=", TLP_GREEN.id)) == [TLP_GREEN]
+
+
+def test_source_mixed(indicator):
+    msrc = MemorySource([TLP_GREEN, indicator])
+
+    assert msrc.get(TLP_GREEN.id) == TLP_GREEN
+    assert msrc.all_versions(TLP_GREEN.id) == [TLP_GREEN]
+    assert msrc.query(Filter("id", "=", TLP_GREEN.id)) == [TLP_GREEN]
+
+    assert msrc.get(indicator.id) == indicator
+    assert msrc.all_versions(indicator.id) == [indicator]
+    assert msrc.query(Filter("id", "=", indicator.id)) == [indicator]
+
+    all_objs = msrc.query()
+    assert TLP_GREEN in all_objs
+    assert indicator in all_objs
+    assert len(all_objs) == 2
+
+
+def test_sink_mixed(indicator):
+    # just make sure there is no crash
+    msink = MemorySink([TLP_GREEN, indicator])
+    msink.add([TLP_GREEN, indicator])
+
+
+def test_store_mixed(indicator):
+    mstore = MemoryStore([TLP_GREEN, indicator])
+
+    assert mstore.get(TLP_GREEN.id) == TLP_GREEN
+    assert mstore.all_versions(TLP_GREEN.id) == [TLP_GREEN]
+    assert mstore.query(Filter("id", "=", TLP_GREEN.id)) == [TLP_GREEN]
+
+    assert mstore.get(indicator.id) == indicator
+    assert mstore.all_versions(indicator.id) == [indicator]
+    assert mstore.query(Filter("id", "=", indicator.id)) == [indicator]
+
+    all_objs = mstore.query()
+    assert TLP_GREEN in all_objs
+    assert indicator in all_objs
+    assert len(all_objs) == 2
diff --git a/stix2/test/v21/test_datastore_memory.py b/stix2/test/v21/test_datastore_memory.py
index 2bc4730..d77c6b4 100644
--- a/stix2/test/v21/test_datastore_memory.py
+++ b/stix2/test/v21/test_datastore_memory.py
@@ -5,6 +5,7 @@ import pytest
 
 from stix2 import Filter, MemorySource, MemoryStore, properties
 from stix2.datastore import make_id
+from stix2.utils import parse_into_datetime
 from stix2.v21 import (
     Bundle, Campaign, CustomObject, Identity, Indicator, Malware, Relationship,
 )
@@ -178,7 +179,7 @@ def test_memory_store_all_versions(mem_store):
     ))
 
     resp = mem_store.all_versions("indicator--00000000-0000-4000-8000-000000000001")
-    assert len(resp) == 1  # MemoryStore can only store 1 version of each object
+    assert len(resp) == 3
 
 
 def test_memory_store_query(mem_store):
@@ -190,25 +191,27 @@ def test_memory_store_query(mem_store):
 def test_memory_store_query_single_filter(mem_store):
     query = Filter('id', '=', 'indicator--00000000-0000-4000-8000-000000000001')
     resp = mem_store.query(query)
-    assert len(resp) == 1
+    assert len(resp) == 2
 
 
 def test_memory_store_query_empty_query(mem_store):
     resp = mem_store.query()
     # sort since returned in random order
-    resp = sorted(resp, key=lambda k: k['id'])
-    assert len(resp) == 2
+    resp = sorted(resp, key=lambda k: (k['id'], k['modified']))
+    assert len(resp) == 3
     assert resp[0]['id'] == 'indicator--00000000-0000-4000-8000-000000000001'
-    assert resp[0]['modified'] == '2017-01-27T13:49:53.936Z'
-    assert resp[1]['id'] == 'indicator--00000000-0000-4000-8000-000000000002'
-    assert resp[1]['modified'] == '2017-01-27T13:49:53.935Z'
+    assert resp[0]['modified'] == parse_into_datetime('2017-01-27T13:49:53.935Z')
+    assert resp[1]['id'] == 'indicator--00000000-0000-4000-8000-000000000001'
+    assert resp[1]['modified'] == parse_into_datetime('2017-01-27T13:49:53.936Z')
+    assert resp[2]['id'] == 'indicator--00000000-0000-4000-8000-000000000002'
+    assert resp[2]['modified'] == parse_into_datetime('2017-01-27T13:49:53.935Z')
 
 
 def test_memory_store_query_multiple_filters(mem_store):
     mem_store.source.filters.add(Filter('type', '=', 'indicator'))
     query = Filter('id', '=', 'indicator--00000000-0000-4000-8000-000000000001')
     resp = mem_store.query(query)
-    assert len(resp) == 1
+    assert len(resp) == 2
 
 
 def test_memory_store_save_load_file(mem_store, fs_mem_store):
@@ -229,12 +232,8 @@ def test_memory_store_save_load_file(mem_store, fs_mem_store):
 
 def test_memory_store_add_invalid_object(mem_store):
     ind = ('indicator', IND1)  # tuple isn't valid
-    with pytest.raises(TypeError) as excinfo:
+    with pytest.raises(TypeError):
         mem_store.add(ind)
-    assert 'stix_data expected to be' in str(excinfo.value)
-    assert 'a python-stix2 object' in str(excinfo.value)
-    assert 'JSON formatted STIX' in str(excinfo.value)
-    assert 'JSON formatted STIX bundle' in str(excinfo.value)
 
 
 def test_memory_store_object_with_custom_property(mem_store):
diff --git a/stix2/test/v21/test_datastore_taxii.py b/stix2/test/v21/test_datastore_taxii.py
index 0ba27b1..9c555ba 100644
--- a/stix2/test/v21/test_datastore_taxii.py
+++ b/stix2/test/v21/test_datastore_taxii.py
@@ -16,8 +16,10 @@ COLLECTION_URL = 'https://example.com/api1/collections/91a7b528-80eb-42ed-a74d-c
 class MockTAXIICollectionEndpoint(Collection):
     """Mock for taxii2_client.TAXIIClient"""
 
-    def __init__(self, url, **kwargs):
-        super(MockTAXIICollectionEndpoint, self).__init__(url, **kwargs)
+    def __init__(self, url, collection_info):
+        super(MockTAXIICollectionEndpoint, self).__init__(
+            url, collection_info=collection_info,
+        )
         self.objects = []
 
     def add_objects(self, bundle):
@@ -69,7 +71,7 @@ class MockTAXIICollectionEndpoint(Collection):
 @pytest.fixture
 def collection(stix_objs1):
     mock = MockTAXIICollectionEndpoint(
-        COLLECTION_URL, **{
+        COLLECTION_URL, {
             "id": "91a7b528-80eb-42ed-a74d-c6fbd5a26116",
             "title": "Writable Collection",
             "description": "This collection is a dropbox for submitting indicators",
@@ -78,7 +80,7 @@ def collection(stix_objs1):
             "media_types": [
                 "application/vnd.oasis.stix+json; version=2.0",
             ],
-        }
+        },
     )
 
     mock.objects.extend(stix_objs1)
@@ -88,7 +90,7 @@ def collection(stix_objs1):
 @pytest.fixture
 def collection_no_rw_access(stix_objs1):
     mock = MockTAXIICollectionEndpoint(
-        COLLECTION_URL, **{
+        COLLECTION_URL, {
             "id": "91a7b528-80eb-42ed-a74d-c6fbd5a26116",
             "title": "Not writeable or readable Collection",
             "description": "This collection is a dropbox for submitting indicators",
@@ -97,7 +99,7 @@ def collection_no_rw_access(stix_objs1):
             "media_types": [
                 "application/vnd.oasis.stix+json; version=2.0",
             ],
-        }
+        },
     )
 
     mock.objects.extend(stix_objs1)
diff --git a/stix2/test/v21/test_environment.py b/stix2/test/v21/test_environment.py
index 21f0d7c..e08971e 100644
--- a/stix2/test/v21/test_environment.py
+++ b/stix2/test/v21/test_environment.py
@@ -131,7 +131,7 @@ def test_environment_functions():
 
     # Get both versions of the object
     resp = env.all_versions(INDICATOR_ID)
-    assert len(resp) == 1  # should be 2, but MemoryStore only keeps 1 version of objects
+    assert len(resp) == 2
 
     # Get just the most recent version of the object
     resp = env.get(INDICATOR_ID)
diff --git a/stix2/test/v21/test_markings.py b/stix2/test/v21/test_markings.py
index 4fb6203..7782236 100644
--- a/stix2/test/v21/test_markings.py
+++ b/stix2/test/v21/test_markings.py
@@ -12,7 +12,7 @@ EXPECTED_TLP_MARKING_DEFINITION = """{
     "type": "marking-definition",
     "spec_version": "2.1",
     "id": "marking-definition--613f2e26-407d-48c7-9eca-b8e91df99dc9",
-    "created": "2017-01-20T00:00:00Z",
+    "created": "2017-01-20T00:00:00.000Z",
     "definition_type": "tlp",
     "definition": {
         "tlp": "white"
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index 3dd3222..87c7225 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -1,6 +1,7 @@
 """STIX 2.1 Common Data Types and Properties."""
 
 from collections import OrderedDict
+import copy
 
 from ..base import _STIXBase
 from ..custom import _custom_marking_builder
@@ -160,6 +161,13 @@ class MarkingDefinition(_STIXBase, _MarkingsMixin):
             except KeyError:
                 raise ValueError("definition_type must be a valid marking type")
 
+            if marking_type == TLPMarking:
+                # TLP instances in the spec have millisecond precision unlike other markings
+                self._properties = copy.deepcopy(self._properties)
+                self._properties.update([
+                    ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
+                ])
+
             if not isinstance(kwargs['definition'], marking_type):
                 defn = _get_dict(kwargs['definition'])
                 kwargs['definition'] = marking_type(**defn)