From 2bfc91ec7567c0c7fbc7c8dd94725422b6f7ea32 Mon Sep 17 00:00:00 2001
From: nightcityblade <nightcityblade@gmail.com>
Date: Fri, 3 Jul 2026 11:26:42 +0800
Subject: [PATCH] Fix datetime last_modified rounding

---
 CHANGES/5303.bugfix.rst    |  3 +++
 aiohttp/web_response.py    |  2 +-
 tests/test_web_response.py | 11 +++++++++++
 3 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 CHANGES/5303.bugfix.rst

diff --git a/CHANGES/5303.bugfix.rst b/CHANGES/5303.bugfix.rst
new file mode 100644
index 00000000000..dd60f3a4dea
--- /dev/null
+++ b/CHANGES/5303.bugfix.rst
@@ -0,0 +1,3 @@
+Aligned ``StreamResponse.last_modified`` datetime handling with the timestamp
+path so sub-second values round up consistently.
+-- by :user:`nightcityblade`.
diff --git a/aiohttp/web_response.py b/aiohttp/web_response.py
index cea31be1733..51a79d7b78c 100644
--- a/aiohttp/web_response.py
+++ b/aiohttp/web_response.py
@@ -372,7 +372,7 @@ def last_modified(
             )
         elif isinstance(value, datetime.datetime):
             self._headers[hdrs.LAST_MODIFIED] = time.strftime(
-                "%a, %d %b %Y %H:%M:%S GMT", value.utctimetuple()
+                "%a, %d %b %Y %H:%M:%S GMT", time.gmtime(math.ceil(value.timestamp()))
             )
         elif isinstance(value, str):
             self._headers[hdrs.LAST_MODIFIED] = value
diff --git a/tests/test_web_response.py b/tests/test_web_response.py
index c5ca849b6b9..cb02535ead3 100644
--- a/tests/test_web_response.py
+++ b/tests/test_web_response.py
@@ -339,6 +339,17 @@ def test_last_modified_datetime() -> None:
     assert resp.last_modified == dt
 
 
+def test_last_modified_datetime_rounds_like_timestamp() -> None:
+    resp = StreamResponse()
+
+    dt = datetime.datetime(2001, 2, 3, 4, 5, 6, 1000, datetime.timezone.utc)
+    resp.last_modified = dt
+
+    assert resp.last_modified == datetime.datetime(
+        2001, 2, 3, 4, 5, 7, tzinfo=datetime.timezone.utc
+    )
+
+
 def test_last_modified_reset() -> None:
     resp = StreamResponse()