diff --git a/bson/__init__.py b/bson/__init__.py
index 4ad98237b6..16573b3de2 100644
--- a/bson/__init__.py
+++ b/bson/__init__.py
@@ -819,7 +819,7 @@ def _dict_to_bson(doc, check_keys, opts, top_level=True):
 def _millis_to_datetime(millis, opts):
     """Convert milliseconds since epoch UTC to datetime."""
     diff = ((millis % 1000) + 1000) % 1000
-    seconds = (millis - diff) / 1000
+    seconds = (millis - diff) // 1000
     micros = diff * 1000
     if opts.tz_aware:
         dt = EPOCH_AWARE + datetime.timedelta(seconds=seconds,
@@ -837,7 +837,7 @@ def _datetime_to_millis(dtm):
     if dtm.utcoffset() is not None:
         dtm = dtm - dtm.utcoffset()
     return int(calendar.timegm(dtm.timetuple()) * 1000 +
-               dtm.microsecond / 1000)
+               dtm.microsecond // 1000)
 
 
 _CODEC_OPTIONS_TYPE_ERROR = TypeError(
diff --git a/test/test_bson.py b/test/test_bson.py
index b5291c5ad5..86653c9c90 100644
--- a/test/test_bson.py
+++ b/test/test_bson.py
@@ -474,6 +474,13 @@ def test_datetime_encode_decode(self):
         dt2 = BSON.encode({"date": dt1}).decode()["date"]
         self.assertEqual(dt1, dt2)
 
+    def test_large_datetime_truncation(self):
+        # Ensure that a large datetime is truncated correctly.
+        dt1 = datetime.datetime(9999, 1, 1, 1, 1, 1, 999999)
+        dt2 = BSON.encode({"date": dt1}).decode()["date"]
+        self.assertEqual(dt2.microsecond, 999000)
+        self.assertEqual(dt2.second, dt1.second)
+
     def test_aware_datetime(self):
         aware = datetime.datetime(1993, 4, 4, 2,
                                   tzinfo=FixedOffset(555, "SomeZone"))