diff --git a/msgpack/ext.py b/msgpack/ext.py
index e5d7d222..7fdad86f 100644
--- a/msgpack/ext.py
+++ b/msgpack/ext.py
@@ -158,7 +158,7 @@ def to_datetime(self):
         """
         utc = datetime.timezone.utc
         return datetime.datetime.fromtimestamp(0, utc) + datetime.timedelta(
-            seconds=self.seconds, microseconds=round(self.nanoseconds / 1e3)
+            seconds=self.seconds, microseconds=self.nanoseconds // 1e3
         )
 
     @staticmethod
diff --git a/test/test_timestamp.py b/test/test_timestamp.py
index 9b3a2606..ce786edb 100644
--- a/test/test_timestamp.py
+++ b/test/test_timestamp.py
@@ -99,11 +99,6 @@ def test_timestamp_datetime():
 
     assert Timestamp.from_datetime(ts).to_datetime() == ts
 
-    t2 = Timestamp(1713256989, 420318123)
-    t3 = Timestamp(1713256989, 420318499)
-    t4 = Timestamp(1713256989, 420318501)
-    assert t2.to_datetime() == t3.to_datetime() != t4.to_datetime()
-
 
 def test_unpack_datetime():
     t = Timestamp(42, 14)