diff --git a/gcloud/datastore/helpers.py b/gcloud/datastore/helpers.py
index ccef721e97b4d..7e5309778f13a 100644
--- a/gcloud/datastore/helpers.py
+++ b/gcloud/datastore/helpers.py
@@ -1,6 +1,6 @@
 """Helper methods for dealing with Cloud Datastore's Protobuf API."""
+import calendar
 from datetime import datetime
-import time
 
 import pytz
 
@@ -37,7 +37,15 @@ def get_protobuf_attribute_and_value(val):
   """
 
   if isinstance(val, datetime):
-    name, value = 'timestamp_microseconds', time.mktime(val.timetuple())
+    name = 'timestamp_microseconds'
+    # If the datetime is naive (no timezone), consider that it was
+    # intended to be UTC and replace the tzinfo to that effect.
+    if not val.tzinfo:
+      val = val.replace(tzinfo=pytz.utc)
+    # Regardless of what timezone is on the value, convert it to UTC.
+    val = val.astimezone(pytz.utc)
+    # Convert the datetime to a microsecond timestamp.
+    value = long(calendar.timegm(val.timetuple()) * 10**6) + val.microsecond
   elif isinstance(val, Key):
     name, value = 'key', val.to_protobuf()
   elif isinstance(val, bool):
diff --git a/gcloud/datastore/test_helpers.py b/gcloud/datastore/test_helpers.py
index 262ebe511a9a3..3e1653bade5fd 100644
--- a/gcloud/datastore/test_helpers.py
+++ b/gcloud/datastore/test_helpers.py
@@ -1,3 +1,4 @@
+import calendar
 from datetime import datetime
 import time
 
@@ -28,7 +29,7 @@ def test_get_protobuf_attribute(self):
             test_value, expected_name, actual_name))
 
   def test_get_protobuf_value(self):
-    now = datetime.now()
+    now = datetime.utcnow()
 
     mapping = (
         (str('string'), 'string'),
@@ -37,7 +38,7 @@ def test_get_protobuf_value(self):
         (long(), int()),
         (float(), float()),
         (bool(), bool()),
-        (now, time.mktime(now.timetuple())),
+        (now, long(calendar.timegm(now.timetuple()) * 10**6 + now.microsecond)),
         (Key(), Key().to_protobuf()),
         )