diff --git a/stix2/markings.py b/stix2/markings.py
index aff2ccb..7742bbf 100644
--- a/stix2/markings.py
+++ b/stix2/markings.py
@@ -1,7 +1,8 @@
 """STIX 2.0 Marking Objects"""
 
 from .base import _STIXBase
-from .properties import IDProperty, TypeProperty, ListProperty, ReferenceProperty, Property, SelectorProperty
+from .properties import (IDProperty, TypeProperty, ListProperty, TimestampProperty,
+                         ReferenceProperty, Property, SelectorProperty)
 from .utils import NOW
 
 
@@ -15,7 +16,7 @@ class GranularMarking(_STIXBase):
 class MarkingDefinition(_STIXBase):
     _type = 'marking-definition'
     _properties = {
-        'created': Property(default=lambda: NOW),
+        'created': TimestampProperty(default=lambda: NOW),
         'external_references': Property(),
         'created_by_ref': ReferenceProperty(type="identity"),
         'object_marking_refs': ListProperty(ReferenceProperty(type="marking-definition")),
diff --git a/stix2/properties.py b/stix2/properties.py
index 1ed09a1..e2ad31e 100644
--- a/stix2/properties.py
+++ b/stix2/properties.py
@@ -206,25 +206,26 @@ class BooleanProperty(Property):
 class TimestampProperty(Property):
 
     def validate(self, value):
-        if isinstance(value, dt.datetime):
-            return value
-        elif isinstance(value, dt.date):
-            return dt.datetime.combine(value, dt.time())
+        if isinstance(value, dt.date):
+            if hasattr(value, 'hour'):
+                return value
+            else:
+                # Add a time component
+                return dt.datetime.combine(value, dt.time(), tzinfo=pytz.timezone('US/Eastern'))
 
+        # value isn't a date or datetime object so assume it's a string
         try:
-            return parser.parse(value).astimezone(pytz.utc)
-        except ValueError:
-            # Doesn't have timezone info in the string
-            try:
-                return pytz.utc.localize(parser.parse(value))
-            except TypeError:
-                # Unknown format
-                raise ValueError("must be a datetime object, date object, or "
-                                 "timestamp string in a recognizable format.")
+            parsed = parser.parse(value)
         except TypeError:
-            # Isn't a string
+            # Unknown format
             raise ValueError("must be a datetime object, date object, or "
-                             "timestamp string.")
+                             "timestamp string in a recognizable format.")
+        if parsed.tzinfo:
+            return parsed.astimezone(pytz.utc)
+        else:
+            # Doesn't have timezone info in the string; assume UTC
+            # TODO Should we default to system local timezone instead?
+            return pytz.utc.localize(parsed)
 
 
 REF_REGEX = re.compile("^[a-z][a-z-]+[a-z]--[0-9a-fA-F]{8}-[0-9a-fA-F]{4}"
diff --git a/stix2/test/test_markings.py b/stix2/test/test_markings.py
index fd9b376..78e42f9 100644
--- a/stix2/test/test_markings.py
+++ b/stix2/test/test_markings.py
@@ -3,7 +3,7 @@ from stix2.markings import TLP_WHITE
 import pytest
 
 EXPECTED_TLP_MARKING_DEFINITION = """{
-    "created": "2017-01-20T00:00:00.000Z",
+    "created": "2017-01-20T00:00:00Z",
     "definition": {
         "tlp": "white"
     },
@@ -13,7 +13,7 @@ EXPECTED_TLP_MARKING_DEFINITION = """{
 }"""
 
 EXPECTED_STATEMENT_MARKING_DEFINITION = """{
-    "created": "2017-01-20T00:00:00.000Z",
+    "created": "2017-01-20T00:00:00Z",
     "definition": {
         "statement": "Copyright 2016, Example Corp"
     },
@@ -33,7 +33,7 @@ EXPECTED_GRANULAR_MARKING = """{
 }"""
 
 EXPECTED_CAMPAIGN_WITH_GRANULAR_MARKINGS = """{
-    "created": "2016-04-06T20:03:00.000Z",
+    "created": "2016-04-06T20:03:00Z",
     "created_by_ref": "identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
     "description": "Campaign by Green Group against a series of targets in the financial services sector.",
     "granular_markings": [
@@ -45,7 +45,7 @@ EXPECTED_CAMPAIGN_WITH_GRANULAR_MARKINGS = """{
         }
     ],
     "id": "campaign--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
-    "modified": "2016-04-06T20:03:00.000Z",
+    "modified": "2016-04-06T20:03:00Z",
     "name": "Green Group Attacks Against Finance",
     "type": "campaign"
 }"""
@@ -100,8 +100,8 @@ def test_campaign_with_granular_markings_example():
     campaign = stix2.Campaign(
         id="campaign--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
         created_by_ref="identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
-        created="2016-04-06T20:03:00.000Z",
-        modified="2016-04-06T20:03:00.000Z",
+        created="2016-04-06T20:03:00Z",
+        modified="2016-04-06T20:03:00Z",
         name="Green Group Attacks Against Finance",
         description="Campaign by Green Group against a series of targets in the financial services sector.",
         granular_markings=[
diff --git a/stix2/utils.py b/stix2/utils.py
index 0acd5de..eafaa76 100644
--- a/stix2/utils.py
+++ b/stix2/utils.py
@@ -1,7 +1,7 @@
 """Utility functions and classes for the stix2 library."""
 
 import datetime as dt
-
+import time
 import pytz
 
 # Sentinel value for fields that should be set to the current time.
@@ -15,12 +15,20 @@ def get_timestamp():
 
 
 def format_datetime(dttm):
-    # TODO: how to handle naive datetime
+    # 1. Convert to timezone-aware
+    # 2. Convert to UTC
+    # 3. Format in ISO format
+    # 4. Add subsecond value if non-zero
+    # 5. Add "Z"
 
-    # 1. Convert to UTC
-    # 2. Format in ISO format
-    # 3. Strip off "+00:00"
-    # 4. Add "Z"
-
-    # TODO: how to handle timestamps with subsecond 0's
-    return dttm.astimezone(pytz.utc).isoformat()[:-6] + "Z"
+    try:
+        zoned = dttm.astimezone(pytz.utc)
+    except ValueError:
+        # dttm is timezone-naive
+        tz_name = time.tzname[time.localtime().tm_isdst]
+        zoned = pytz.timezone(tz_name).localize(dttm).astimezone(pytz.utc)
+    ts = zoned.strftime("%Y-%m-%dT%H:%M:%S")
+    if zoned.microsecond > 0:
+        ms = zoned.strftime("%f")
+        ts = ts + '.' + ms.rstrip("0")
+    return ts + "Z"