diff --git a/setup.py b/setup.py
index 4c15871..eff4c2b 100644
--- a/setup.py
+++ b/setup.py
@@ -4,6 +4,7 @@ from setuptools import setup, find_packages
 install_requires = [
     'pytz',
     'six',
+    'python-dateutil',
 ]
 
 setup(
diff --git a/stix2/common.py b/stix2/common.py
index 3a77d0b..2a6479a 100644
--- a/stix2/common.py
+++ b/stix2/common.py
@@ -2,13 +2,13 @@
 
 from .base import _STIXBase
 from .properties import (Property, BooleanProperty, ReferenceProperty,
-                         StringProperty)
+                         StringProperty, TimestampProperty)
 from .utils import NOW
 
 COMMON_PROPERTIES = {
     # 'type' and 'id' should be defined on each individual type
-    'created': Property(default=lambda: NOW),
-    'modified': Property(default=lambda: NOW),
+    'created': TimestampProperty(default=lambda: NOW),
+    'modified': TimestampProperty(default=lambda: NOW),
     'external_references': Property(),
     'revoked': BooleanProperty(),
     'created_by_ref': ReferenceProperty(),
diff --git a/stix2/properties.py b/stix2/properties.py
index 7dab517..fe5e520 100644
--- a/stix2/properties.py
+++ b/stix2/properties.py
@@ -1,6 +1,9 @@
 import re
 import uuid
 from six import PY2
+import datetime as dt
+import pytz
+from dateutil import parser
 
 
 class Property(object):
@@ -194,6 +197,30 @@ class BooleanProperty(Property):
             raise ValueError("must be a boolean value.")
 
 
+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())
+
+        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.")
+        except TypeError:
+            # Isn't a string
+            raise ValueError("must be a datetime object, date object, or "
+                             "timestamp string.")
+
+
 REF_REGEX = re.compile("^[a-z][a-z-]+[a-z]--[0-9a-fA-F]{8}-[0-9a-fA-F]{4}"
                        "-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$")
 
diff --git a/stix2/test/test_attack_pattern.py b/stix2/test/test_attack_pattern.py
index a9d53b6..36b5b6d 100644
--- a/stix2/test/test_attack_pattern.py
+++ b/stix2/test/test_attack_pattern.py
@@ -1,7 +1,7 @@
 import stix2
 
 EXPECTED = """{
-    "created": "2016-05-12T08:17:27.000Z",
+    "created": "2016-05-12T08:17:27Z",
     "description": "...",
     "external_references": [
         {
@@ -10,7 +10,7 @@ EXPECTED = """{
         }
     ],
     "id": "attack-pattern--0c7b5b88-8ff7-4a4d-aa9d-feb398cd0061",
-    "modified": "2016-05-12T08:17:27.000Z",
+    "modified": "2016-05-12T08:17:27Z",
     "name": "Spear Phishing",
     "type": "attack-pattern"
 }"""
@@ -19,8 +19,8 @@ EXPECTED = """{
 def test_attack_pattern_example():
     ap = stix2.AttackPattern(
         id="attack-pattern--0c7b5b88-8ff7-4a4d-aa9d-feb398cd0061",
-        created="2016-05-12T08:17:27.000Z",
-        modified="2016-05-12T08:17:27.000Z",
+        created="2016-05-12T08:17:27Z",
+        modified="2016-05-12T08:17:27Z",
         name="Spear Phishing",
         external_references=[{
             "source_name": "capec",
diff --git a/stix2/test/test_campaign.py b/stix2/test/test_campaign.py
index ce68946..741f5ba 100644
--- a/stix2/test/test_campaign.py
+++ b/stix2/test/test_campaign.py
@@ -1,11 +1,11 @@
 import stix2
 
 EXPECTED = """{
-    "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.",
     "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"
 }"""
@@ -15,8 +15,8 @@ def test_campaign_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."
     )
diff --git a/stix2/test/test_course_of_action.py b/stix2/test/test_course_of_action.py
index 8c64cde..d35680e 100644
--- a/stix2/test/test_course_of_action.py
+++ b/stix2/test/test_course_of_action.py
@@ -1,11 +1,11 @@
 import stix2
 
 EXPECTED = """{
-    "created": "2016-04-06T20:03:48.000Z",
+    "created": "2016-04-06T20:03:48Z",
     "created_by_ref": "identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
     "description": "This is how to add a filter rule to block inbound access to TCP port 80 to the existing UDP 1434 filter ...",
     "id": "course-of-action--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
-    "modified": "2016-04-06T20:03:48.000Z",
+    "modified": "2016-04-06T20:03:48Z",
     "name": "Add TCP port 80 Filter Rule to the existing Block UDP 1434 Filter",
     "type": "course-of-action"
 }"""
@@ -15,8 +15,8 @@ def test_course_of_action_example():
     coa = stix2.CourseOfAction(
         id="course-of-action--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
         created_by_ref="identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
-        created="2016-04-06T20:03:48.000Z",
-        modified="2016-04-06T20:03:48.000Z",
+        created="2016-04-06T20:03:48Z",
+        modified="2016-04-06T20:03:48Z",
         name="Add TCP port 80 Filter Rule to the existing Block UDP 1434 Filter",
         description="This is how to add a filter rule to block inbound access to TCP port 80 to the existing UDP 1434 filter ..."
     )
diff --git a/stix2/test/test_identity.py b/stix2/test/test_identity.py
index 632f229..e43614e 100644
--- a/stix2/test/test_identity.py
+++ b/stix2/test/test_identity.py
@@ -1,10 +1,10 @@
 import stix2
 
 EXPECTED = """{
-    "created": "2015-12-21T19:59:11.000Z",
+    "created": "2015-12-21T19:59:11Z",
     "id": "identity--311b2d2d-f010-5473-83ec-1edf84858f4c",
     "identity_class": "individual",
-    "modified": "2015-12-21T19:59:11.000Z",
+    "modified": "2015-12-21T19:59:11Z",
     "name": "John Smith",
     "type": "identity"
 }"""
@@ -13,8 +13,8 @@ EXPECTED = """{
 def test_identity_example():
     report = stix2.Identity(
         id="identity--311b2d2d-f010-5473-83ec-1edf84858f4c",
-        created="2015-12-21T19:59:11.000Z",
-        modified="2015-12-21T19:59:11.000Z",
+        created="2015-12-21T19:59:11Z",
+        modified="2015-12-21T19:59:11Z",
         name="John Smith",
         identity_class="individual",
     )
diff --git a/stix2/test/test_intrusion_set.py b/stix2/test/test_intrusion_set.py
index ba158d7..e60532f 100644
--- a/stix2/test/test_intrusion_set.py
+++ b/stix2/test/test_intrusion_set.py
@@ -4,7 +4,7 @@ EXPECTED = """{
     "aliases": [
         "Zookeeper"
     ],
-    "created": "2016-04-06T20:03:48.000Z",
+    "created": "2016-04-06T20:03:48Z",
     "created_by_ref": "identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
     "description": "Incidents usually feature a shared TTP of a bobcat being released...",
     "goals": [
@@ -13,7 +13,7 @@ EXPECTED = """{
         "damage"
     ],
     "id": "intrusion-set--4e78f46f-a023-4e5f-bc24-71b3ca22ec29",
-    "modified": "2016-04-06T20:03:48.000Z",
+    "modified": "2016-04-06T20:03:48Z",
     "name": "Bobcat Breakin",
     "type": "intrusion-set"
 }"""
@@ -23,8 +23,8 @@ def test_intrusion_set_example():
     intrusion_set = stix2.IntrusionSet(
         id="intrusion-set--4e78f46f-a023-4e5f-bc24-71b3ca22ec29",
         created_by_ref="identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
-        created="2016-04-06T20:03:48.000Z",
-        modified="2016-04-06T20:03:48.000Z",
+        created="2016-04-06T20:03:48Z",
+        modified="2016-04-06T20:03:48Z",
         name="Bobcat Breakin",
         description="Incidents usually feature a shared TTP of a bobcat being released...",
         aliases=["Zookeeper"],
diff --git a/stix2/test/test_malware.py b/stix2/test/test_malware.py
index c80c28e..9537840 100644
--- a/stix2/test/test_malware.py
+++ b/stix2/test/test_malware.py
@@ -107,8 +107,8 @@ def test_parse_malware(data):
 
     assert mal.type == 'malware'
     assert mal.id == MALWARE_ID
-    assert mal.created == "2016-05-12T08:17:27Z"
-    assert mal.modified == "2016-05-12T08:17:27Z"
+    assert mal.created == dt.datetime(2016, 5, 12, 8, 17, 27, tzinfo=pytz.utc)
+    assert mal.modified == dt.datetime(2016, 5, 12, 8, 17, 27, tzinfo=pytz.utc)
     assert mal.labels == ['ransomware']
     assert mal.name == "Cryptolocker"
 
diff --git a/stix2/test/test_observed_data.py b/stix2/test/test_observed_data.py
index 6fd95a2..f47e43e 100644
--- a/stix2/test/test_observed_data.py
+++ b/stix2/test/test_observed_data.py
@@ -1,12 +1,12 @@
 import stix2
 
 EXPECTED = """{
-    "created": "2016-04-06T19:58:16.000Z",
+    "created": "2016-04-06T19:58:16Z",
     "created_by_ref": "identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
     "first_observed": "2015-12-21T19:00:00Z",
     "id": "observed-data--b67d30ff-02ac-498a-92f9-32f845f448cf",
     "last_observed": "2015-12-21T19:00:00Z",
-    "modified": "2016-04-06T19:58:16.000Z",
+    "modified": "2016-04-06T19:58:16Z",
     "number_observed": 50,
     "objects": {
         "0": {
@@ -21,8 +21,8 @@ def test_observed_data_example():
     observed_data = stix2.ObservedData(
         id="observed-data--b67d30ff-02ac-498a-92f9-32f845f448cf",
         created_by_ref="identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
-        created="2016-04-06T19:58:16.000Z",
-        modified="2016-04-06T19:58:16.000Z",
+        created="2016-04-06T19:58:16Z",
+        modified="2016-04-06T19:58:16Z",
         first_observed="2015-12-21T19:00:00Z",
         last_observed="2015-12-21T19:00:00Z",
         number_observed=50,
diff --git a/stix2/test/test_properties.py b/stix2/test/test_properties.py
index dad250c..0bbcae1 100644
--- a/stix2/test/test_properties.py
+++ b/stix2/test/test_properties.py
@@ -2,7 +2,8 @@ import pytest
 
 from stix2.properties import (Property, BooleanProperty, ListProperty,
                               StringProperty, TypeProperty, IDProperty,
-                              ReferenceProperty)
+                              ReferenceProperty, TimestampProperty)
+from .constants import FAKE_TIME
 
 
 def test_property():
@@ -131,3 +132,21 @@ def test_reference_property():
     assert ref_prop.validate("my-type--3a331bfe-0566-55e1-a4a0-9a2cd355a300")
     with pytest.raises(ValueError):
         ref_prop.validate("foo")
+
+
+@pytest.mark.parametrize("value", [
+    '2017-01-01T12:34:56Z',
+    '2017-01-01 12:34:56',
+    'Jan 1 2017 12:34:56',
+])
+def test_timestamp_property_valid(value):
+    ts_prop = TimestampProperty()
+    assert ts_prop.validate(value) == FAKE_TIME
+
+
+def test_timestamp_property_invalid():
+    ts_prop = TimestampProperty()
+    with pytest.raises(ValueError):
+        ts_prop.validate(1)
+    with pytest.raises(ValueError):
+        ts_prop.validate("someday sometime")
diff --git a/stix2/test/test_report.py b/stix2/test/test_report.py
index acd4e30..2727dbc 100644
--- a/stix2/test/test_report.py
+++ b/stix2/test/test_report.py
@@ -1,14 +1,14 @@
 import stix2
 
 EXPECTED = """{
-    "created": "2015-12-21T19:59:11.000Z",
+    "created": "2015-12-21T19:59:11Z",
     "created_by_ref": "identity--a463ffb3-1bd9-4d94-b02d-74e4f1658283",
     "description": "A simple report with an indicator and campaign",
     "id": "report--84e4d88f-44ea-4bcd-bbf3-b2c1c320bcb3",
     "labels": [
         "campaign"
     ],
-    "modified": "2015-12-21T19:59:11.000Z",
+    "modified": "2015-12-21T19:59:11Z",
     "name": "The Black Vine Cyberespionage Group",
     "object_refs": [
         "indicator--26ffb872-1dd9-446e-b6f5-d58527e5b5d2",
@@ -24,8 +24,8 @@ def test_report_example():
     report = stix2.Report(
         id="report--84e4d88f-44ea-4bcd-bbf3-b2c1c320bcb3",
         created_by_ref="identity--a463ffb3-1bd9-4d94-b02d-74e4f1658283",
-        created="2015-12-21T19:59:11.000Z",
-        modified="2015-12-21T19:59:11.000Z",
+        created="2015-12-21T19:59:11Z",
+        modified="2015-12-21T19:59:11Z",
         name="The Black Vine Cyberespionage Group",
         description="A simple report with an indicator and campaign",
         published="2016-01-201T17:00:00Z",
diff --git a/stix2/test/test_threat_actor.py b/stix2/test/test_threat_actor.py
index 8006ac3..c958f80 100644
--- a/stix2/test/test_threat_actor.py
+++ b/stix2/test/test_threat_actor.py
@@ -1,14 +1,14 @@
 import stix2
 
 EXPECTED = """{
-    "created": "2016-04-06T20:03:48.000Z",
+    "created": "2016-04-06T20:03:48Z",
     "created_by_ref": "identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
     "description": "The Evil Org threat actor group",
     "id": "threat-actor--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
     "labels": [
         "crime-syndicate"
     ],
-    "modified": "2016-04-06T20:03:48.000Z",
+    "modified": "2016-04-06T20:03:48Z",
     "name": "Evil Org",
     "type": "threat-actor"
 }"""
@@ -18,8 +18,8 @@ def test_threat_actor_example():
     threat_actor = stix2.ThreatActor(
         id="threat-actor--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
         created_by_ref="identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
-        created="2016-04-06T20:03:48.000Z",
-        modified="2016-04-06T20:03:48.000Z",
+        created="2016-04-06T20:03:48Z",
+        modified="2016-04-06T20:03:48Z",
         name="Evil Org",
         description="The Evil Org threat actor group",
         labels=["crime-syndicate"],
diff --git a/stix2/test/test_tool.py b/stix2/test/test_tool.py
index 201d333..322ff11 100644
--- a/stix2/test/test_tool.py
+++ b/stix2/test/test_tool.py
@@ -1,13 +1,13 @@
 import stix2
 
 EXPECTED = """{
-    "created": "2016-04-06T20:03:48.000Z",
+    "created": "2016-04-06T20:03:48Z",
     "created_by_ref": "identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
     "id": "tool--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
     "labels": [
         "remote-access"
     ],
-    "modified": "2016-04-06T20:03:48.000Z",
+    "modified": "2016-04-06T20:03:48Z",
     "name": "VNC",
     "type": "tool"
 }"""
@@ -17,8 +17,8 @@ def test_tool_example():
     tool = stix2.Tool(
         id="tool--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
         created_by_ref="identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
-        created="2016-04-06T20:03:48.000Z",
-        modified="2016-04-06T20:03:48.000Z",
+        created="2016-04-06T20:03:48Z",
+        modified="2016-04-06T20:03:48Z",
         name="VNC",
         labels=["remote-access"],
     )
diff --git a/stix2/test/test_vulnerability.py b/stix2/test/test_vulnerability.py
index 3daebd9..ee84533 100644
--- a/stix2/test/test_vulnerability.py
+++ b/stix2/test/test_vulnerability.py
@@ -1,7 +1,7 @@
 import stix2
 
 EXPECTED = """{
-    "created": "2016-05-12T08:17:27.000Z",
+    "created": "2016-05-12T08:17:27Z",
     "external_references": [
         {
             "external_id": "CVE-2016-1234",
@@ -9,7 +9,7 @@ EXPECTED = """{
         }
     ],
     "id": "vulnerability--0c7b5b88-8ff7-4a4d-aa9d-feb398cd0061",
-    "modified": "2016-05-12T08:17:27.000Z",
+    "modified": "2016-05-12T08:17:27Z",
     "name": "CVE-2016-1234",
     "type": "vulnerability"
 }"""
@@ -18,8 +18,8 @@ EXPECTED = """{
 def test_vulnerability_example():
     vulnerability = stix2.Vulnerability(
         id="vulnerability--0c7b5b88-8ff7-4a4d-aa9d-feb398cd0061",
-        created="2016-05-12T08:17:27.000Z",
-        modified="2016-05-12T08:17:27.000Z",
+        created="2016-05-12T08:17:27Z",
+        modified="2016-05-12T08:17:27Z",
         name="CVE-2016-1234",
         external_references=[
             stix2.ExternalReference(source_name='cve',