diff --git a/moto/core/utils.py b/moto/core/utils.py
index fb333ccdc..98d6bdc23 100644
--- a/moto/core/utils.py
+++ b/moto/core/utils.py
@@ -80,8 +80,8 @@ class convert_flask_to_httpretty_response(object):
         return response, status, headers
 
 
-def iso_8601_datetime(datetime):
-    return datetime.strftime("%Y-%m-%dT%H:%M:%SZ")
+def iso_8601_datetime_with_milliseconds(datetime):
+    return datetime.strftime("%Y-%m-%dT%H:%M:%S.%f")[:-3] + 'Z'
 
 
 def rfc_1123_datetime(datetime):
diff --git a/moto/s3/models.py b/moto/s3/models.py
index 6e003edda..bbfd571bc 100644
--- a/moto/s3/models.py
+++ b/moto/s3/models.py
@@ -9,7 +9,7 @@ import codecs
 import six
 
 from moto.core import BaseBackend
-from moto.core.utils import iso_8601_datetime, rfc_1123_datetime
+from moto.core.utils import iso_8601_datetime_with_milliseconds, rfc_1123_datetime
 from .exceptions import BucketAlreadyExists, MissingBucket
 from .utils import clean_key_name, _VersionedKeyStore
 
@@ -71,7 +71,7 @@ class FakeKey(object):
 
     @property
     def last_modified_ISO8601(self):
-        return iso_8601_datetime(self.last_modified)
+        return iso_8601_datetime_with_milliseconds(self.last_modified)
 
     @property
     def last_modified_RFC1123(self):
diff --git a/moto/sns/models.py b/moto/sns/models.py
index 4897c5762..b51d197c3 100644
--- a/moto/sns/models.py
+++ b/moto/sns/models.py
@@ -9,7 +9,7 @@ import six
 
 from moto.compat import OrderedDict
 from moto.core import BaseBackend
-from moto.core.utils import iso_8601_datetime
+from moto.core.utils import iso_8601_datetime_with_milliseconds
 from moto.sqs import sqs_backends
 from .utils import make_arn_for_topic, make_arn_for_subscription
 
@@ -69,7 +69,7 @@ class Subscription(object):
             "TopicArn": self.topic.arn,
             "Subject": "my subject",
             "Message": message,
-            "Timestamp": iso_8601_datetime(datetime.datetime.now()),
+            "Timestamp": iso_8601_datetime_with_milliseconds(datetime.datetime.now()),
             "SignatureVersion": "1",
             "Signature": "EXAMPLElDMXvB8r9R83tGoNn0ecwd5UjllzsvSvbItzfaMpN2nk5HVSw7XnOn/49IkxDKz8YrlH2qJXj2iZB0Zo2O71c4qQk1fMUDi3LGpij7RCW7AW9vYYsSqIKRnFS94ilu7NFhUzLiieYr4BKHpdTmdD6c0esKEYBpabxDSc=",
             "SigningCertURL": "https://sns.us-east-1.amazonaws.com/SimpleNotificationService-f3ecfb7224c7233fe7bb5f59f96de52f.pem",
diff --git a/moto/sts/models.py b/moto/sts/models.py
index 3ea6cbd05..224512963 100644
--- a/moto/sts/models.py
+++ b/moto/sts/models.py
@@ -1,7 +1,7 @@
 from __future__ import unicode_literals
 import datetime
 from moto.core import BaseBackend
-from moto.core.utils import iso_8601_datetime
+from moto.core.utils import iso_8601_datetime_with_milliseconds
 
 
 class Token(object):
@@ -13,7 +13,7 @@ class Token(object):
 
     @property
     def expiration_ISO8601(self):
-        return iso_8601_datetime(self.expiration)
+        return iso_8601_datetime_with_milliseconds(self.expiration)
 
 
 class AssumedRole(object):
@@ -27,7 +27,7 @@ class AssumedRole(object):
 
     @property
     def expiration_ISO8601(self):
-        return iso_8601_datetime(self.expiration)
+        return iso_8601_datetime_with_milliseconds(self.expiration)
 
 
 class STSBackend(BaseBackend):
diff --git a/tests/test_ec2/test_spot_instances.py b/tests/test_ec2/test_spot_instances.py
index d914b663b..52e430c15 100644
--- a/tests/test_ec2/test_spot_instances.py
+++ b/tests/test_ec2/test_spot_instances.py
@@ -6,7 +6,7 @@ import sure  # noqa
 
 from moto import mock_ec2
 from moto.backends import get_model
-from moto.core.utils import iso_8601_datetime
+from moto.core.utils import iso_8601_datetime_with_milliseconds
 
 
 @mock_ec2
@@ -16,8 +16,8 @@ def test_request_spot_instances():
     conn.create_security_group('group1', 'description')
     conn.create_security_group('group2', 'description')
 
-    start = iso_8601_datetime(datetime.datetime(2013, 1, 1))
-    end = iso_8601_datetime(datetime.datetime(2013, 1, 2))
+    start = iso_8601_datetime_with_milliseconds(datetime.datetime(2013, 1, 1))
+    end = iso_8601_datetime_with_milliseconds(datetime.datetime(2013, 1, 2))
 
     request = conn.request_spot_instances(
         price=0.5, image_id='ami-abcd1234', count=1, type='one-time',
@@ -145,7 +145,7 @@ def test_tag_spot_instance_request():
     request = requests[0]
 
     tag_dict = dict(request.tags)
-    tag_dict.should.equal({'tag1' : 'value1', 'tag2' : 'value2'})
+    tag_dict.should.equal({'tag1': 'value1', 'tag2': 'value2'})
 
 
 @mock_ec2
@@ -161,7 +161,7 @@ def test_get_all_spot_instance_requests_filtering():
     request2 = conn.request_spot_instances(
         price=0.5, image_id='ami-abcd1234',
     )
-    request3 = conn.request_spot_instances(
+    conn.request_spot_instances(
         price=0.5, image_id='ami-abcd1234',
     )
     request1[0].add_tag('tag1', 'value1')
@@ -169,16 +169,16 @@ def test_get_all_spot_instance_requests_filtering():
     request2[0].add_tag('tag1', 'value1')
     request2[0].add_tag('tag2', 'wrong')
 
-    requests = conn.get_all_spot_instance_requests(filters={'state' : 'active'})
+    requests = conn.get_all_spot_instance_requests(filters={'state': 'active'})
     requests.should.have.length_of(0)
 
-    requests = conn.get_all_spot_instance_requests(filters={'state' : 'open'})
+    requests = conn.get_all_spot_instance_requests(filters={'state': 'open'})
     requests.should.have.length_of(3)
 
-    requests = conn.get_all_spot_instance_requests(filters={'tag:tag1' : 'value1'})
+    requests = conn.get_all_spot_instance_requests(filters={'tag:tag1': 'value1'})
     requests.should.have.length_of(2)
 
-    requests = conn.get_all_spot_instance_requests(filters={'tag:tag1' : 'value1', 'tag:tag2' : 'value2'})
+    requests = conn.get_all_spot_instance_requests(filters={'tag:tag1': 'value1', 'tag:tag2': 'value2'})
     requests.should.have.length_of(1)
 
 
diff --git a/tests/test_s3/test_s3.py b/tests/test_s3/test_s3.py
index 4f09c406f..4d7df5e05 100644
--- a/tests/test_s3/test_s3.py
+++ b/tests/test_s3/test_s3.py
@@ -273,7 +273,7 @@ def test_last_modified():
     key.set_contents_from_string("some value")
 
     rs = bucket.get_all_keys()
-    rs[0].last_modified.should.equal('2012-01-01T12:00:00Z')
+    rs[0].last_modified.should.equal('2012-01-01T12:00:00.000Z')
 
     bucket.get_key("the-key").last_modified.should.equal('Sun, 01 Jan 2012 12:00:00 GMT')
 
diff --git a/tests/test_s3bucket_path/test_s3bucket_path.py b/tests/test_s3bucket_path/test_s3bucket_path.py
index ed26805bf..9b33efe89 100644
--- a/tests/test_s3bucket_path/test_s3bucket_path.py
+++ b/tests/test_s3bucket_path/test_s3bucket_path.py
@@ -133,7 +133,7 @@ def test_last_modified():
     key.set_contents_from_string("some value")
 
     rs = bucket.get_all_keys()
-    rs[0].last_modified.should.equal('2012-01-01T12:00:00Z')
+    rs[0].last_modified.should.equal('2012-01-01T12:00:00.000Z')
 
     bucket.get_key("the-key").last_modified.should.equal('Sun, 01 Jan 2012 12:00:00 GMT')
 
diff --git a/tests/test_sns/test_publishing.py b/tests/test_sns/test_publishing.py
index 7566f8d4d..a7d051b63 100644
--- a/tests/test_sns/test_publishing.py
+++ b/tests/test_sns/test_publishing.py
@@ -75,7 +75,7 @@ def test_publish_to_http():
         "TopicArn": ["arn:aws:sns:us-east-1:123456789012:some-topic"],
         "Subject": ["my subject"],
         "Message": ["my message"],
-        "Timestamp": ["2013-01-01T00:00:00Z"],
+        "Timestamp": ["2013-01-01T00:00:00.000Z"],
         "SignatureVersion": ["1"],
         "Signature": ["EXAMPLElDMXvB8r9R83tGoNn0ecwd5UjllzsvSvbItzfaMpN2nk5HVSw7XnOn/49IkxDKz8YrlH2qJXj2iZB0Zo2O71c4qQk1fMUDi3LGpij7RCW7AW9vYYsSqIKRnFS94ilu7NFhUzLiieYr4BKHpdTmdD6c0esKEYBpabxDSc="],
         "SigningCertURL": ["https://sns.us-east-1.amazonaws.com/SimpleNotificationService-f3ecfb7224c7233fe7bb5f59f96de52f.pem"],
diff --git a/tests/test_sts/test_sts.py b/tests/test_sts/test_sts.py
index dbc0bc439..067ebcce2 100644
--- a/tests/test_sts/test_sts.py
+++ b/tests/test_sts/test_sts.py
@@ -14,7 +14,7 @@ def test_get_session_token():
     conn = boto.connect_sts()
     token = conn.get_session_token(duration=123)
 
-    token.expiration.should.equal('2012-01-01T12:02:03Z')
+    token.expiration.should.equal('2012-01-01T12:02:03.000Z')
     token.session_token.should.equal("AQoEXAMPLEH4aoAH0gNCAPyJxz4BlCFFxWNE1OPTgk5TthT+FvwqnKwRcOIfrRh3c/LTo6UDdyJwOOvEVPvLXCrrrUtdnniCEXAMPLE/IvU1dYUg2RVAJBanLiHb4IgRmpRV3zrkuWJOgQs8IZZaIv2BXIa2R4OlgkBN9bkUDNCJiBeb/AXlzBBko7b15fjrBs2+cTQtpZ3CYWFXG8C5zqx37wnOE49mRl/+OtkIKGO7fAE")
     token.access_key.should.equal("AKIAIOSFODNN7EXAMPLE")
     token.secret_key.should.equal("wJalrXUtnFEMI/K7MDENG/bPxRfiCYzEXAMPLEKEY")
@@ -26,7 +26,7 @@ def test_get_federation_token():
     conn = boto.connect_sts()
     token = conn.get_federation_token(duration=123, name="Bob")
 
-    token.credentials.expiration.should.equal('2012-01-01T12:02:03Z')
+    token.credentials.expiration.should.equal('2012-01-01T12:02:03.000Z')
     token.credentials.session_token.should.equal("AQoDYXdzEPT//////////wEXAMPLEtc764bNrC9SAPBSM22wDOk4x4HIZ8j4FZTwdQWLWsKWHGBuFqwAeMicRXmxfpSPfIeoIYRqTflfKD8YUuwthAx7mSEI/qkPpKPi/kMcGdQrmGdeehM4IC1NtBmUpp2wUE8phUZampKsburEDy0KPkyQDYwT7WZ0wq5VSXDvp75YU9HFvlRd8Tx6q6fE8YQcHNVXAkiY9q6d+xo0rKwT38xVqr7ZD0u0iPPkUL64lIZbqBAz+scqKmlzm8FDrypNC9Yjc8fPOLn9FX9KSYvKTr4rvx3iSIlTJabIQwj2ICCR/oLxBA==")
     token.credentials.access_key.should.equal("AKIAIOSFODNN7EXAMPLE")
     token.credentials.secret_key.should.equal("wJalrXUtnFEMI/K7MDENG/bPxRfiCYzEXAMPLEKEY")
@@ -57,7 +57,7 @@ def test_assume_role():
     role = conn.assume_role(s3_role, "session-name", policy, duration_seconds=123)
 
     credentials = role.credentials
-    credentials.expiration.should.equal('2012-01-01T12:02:03Z')
+    credentials.expiration.should.equal('2012-01-01T12:02:03.000Z')
     credentials.session_token.should.equal("BQoEXAMPLEH4aoAH0gNCAPyJxz4BlCFFxWNE1OPTgk5TthT+FvwqnKwRcOIfrRh3c/LTo6UDdyJwOOvEVPvLXCrrrUtdnniCEXAMPLE/IvU1dYUg2RVAJBanLiHb4IgRmpRV3zrkuWJOgQs8IZZaIv2BXIa2R4OlgkBN9bkUDNCJiBeb/AXlzBBko7b15fjrBs2+cTQtpZ3CYWFXG8C5zqx37wnOE49mRl/+OtkIKGO7fAE")
     credentials.access_key.should.equal("AKIAIOSFODNN7EXAMPLE")
     credentials.secret_key.should.equal("aJalrXUtnFEMI/K7MDENG/bPxRfiCYzEXAMPLEKEY")