diff --git a/stix2/datastore/filesystem.py b/stix2/datastore/filesystem.py
index 07011f7..b23b2c8 100644
--- a/stix2/datastore/filesystem.py
+++ b/stix2/datastore/filesystem.py
@@ -6,6 +6,7 @@ Python STIX 2.0 FileSystem Source/Sink
 import errno
 import json
 import os
+import pytz
 import stat
 
 import six
@@ -14,7 +15,7 @@ from stix2.base import _STIXBase
 from stix2.core import Bundle, parse
 from stix2.datastore import DataSink, DataSource, DataStoreMixin
 from stix2.datastore.filters import Filter, FilterSet, apply_common_filters
-from stix2.utils import deduplicate, is_marking
+from stix2.utils import is_marking, get_type_from_id
 
 
 def _timestamp2filename(timestamp):
@@ -25,20 +26,14 @@ def _timestamp2filename(timestamp):
     :param timestamp: A timestamp, as a datetime.datetime object.
     """
     # Different times will only produce different file names if all timestamps
-    # are in the same time zone!  (Should I convert to UTC just to be safe?)
+    # are in the same time zone!  So if timestamp is timezone-aware convert
+    # to UTC just to be safe.  If naive, just use as-is.
+    if timestamp.tzinfo is not None:
+        timestamp = timestamp.astimezone(pytz.utc)
+
     return timestamp.strftime("%Y%m%d%H%M%S%f")
 
 
-def _type_from_id(id_):
-    """Extract the type from a STIX identifier"""
-    dd_idx = id_.find("--")
-    if dd_idx == -1:
-        raise Exception(
-            "Invalid ID: {}.  Must have format <type>--<uuid>.".format(id_)
-        )
-    return id_[:dd_idx]
-
-
 class AuthSet(object):
     """
     Represents either a whitelist or blacklist of values, where/what we
@@ -164,13 +159,13 @@ def _find_search_optimizations(filters):
                 # contain types within them.
                 allowed_ids = _update_allow(allowed_ids, filter_.value)
                 allowed_types = _update_allow(allowed_types,
-                                              _type_from_id(filter_.value))
+                                              get_type_from_id(filter_.value))
             elif filter_.op == "!=":
                 prohibited_ids.add(filter_.value)
             elif filter_.op == "in":
                 allowed_ids = _update_allow(allowed_ids, filter_.value)
                 allowed_types = _update_allow(allowed_types, (
-                    _type_from_id(id_) for id_ in filter_.value
+                    get_type_from_id(id_) for id_ in filter_.value
                 ))
 
     opt_types = AuthSet(allowed_types, prohibited_types)
@@ -185,12 +180,12 @@ def _find_search_optimizations(filters):
             opt_ids.auth_type == AuthSet.WHITE:
 
         opt_types.values.intersection_update(
-            _type_from_id(id_) for id_ in opt_ids.values
+            get_type_from_id(id_) for id_ in opt_ids.values
         )
 
         opt_ids.values.intersection_update(
             id_ for id_ in opt_ids.values
-            if _type_from_id(id_) in opt_types.values
+            if get_type_from_id(id_) in opt_types.values
         )
 
     return opt_types, opt_ids
@@ -433,10 +428,10 @@ class FileSystemSink(DataSink):
         """
         type_dir = os.path.join(self._stix_dir, stix_obj["type"])
         if is_marking(stix_obj):
-            filename = stix_obj.id
+            filename = stix_obj["id"]
             obj_dir = type_dir
         else:
-            filename = _timestamp2filename(stix_obj.modified)
+            filename = _timestamp2filename(stix_obj["modified"])
             obj_dir = os.path.join(type_dir, stix_obj["id"])
 
         file_path = os.path.join(obj_dir, filename + ".json")
diff --git a/stix2/test/test_datastore_filesystem.py b/stix2/test/test_datastore_filesystem.py
index bab11b3..fafa610 100644
--- a/stix2/test/test_datastore_filesystem.py
+++ b/stix2/test/test_datastore_filesystem.py
@@ -1,6 +1,8 @@
+import datetime
 import errno
 import json
 import os
+import pytz
 import shutil
 import stat
 
@@ -926,3 +928,26 @@ def test_search_auth_set_black_empty(rel_fs_store):
         "relationship",
         "tool"
     }
+
+
+def test_timestamp2filename_naive():
+    dt = datetime.datetime(
+        2010, 6, 15,
+        8, 30, 10, 1234
+    )
+
+    filename = _timestamp2filename(dt)
+    assert filename == "20100615083010001234"
+
+
+def test_timestamp2filename_tz():
+    # one hour west of UTC (i.e. an hour earlier)
+    tz = pytz.FixedOffset(-60)
+    dt = datetime.datetime(
+        2010, 6, 15,
+        7, 30, 10, 1234,
+        tz
+    )
+
+    filename = _timestamp2filename(dt)
+    assert filename == "20100615083010001234"