On the UTC concern: datetime.now(utc).astimezone() does NOT produce UTC timestamps. The .astimezone() call (with no argument) converts to the system's local timezone, correctly reflecting DST. So the flow is: get current time in UTC → convert to local timezone (with DST awareness). The old code used time.timezone which is a module-level constant that never changes, even during DST transitions.

Got it. But, it is creating 2 datetime objects in the process. Do you think my original suggestion could be a better option (assuming it works correctly)? A quick test suggests this version is ~25% slower (and given the frequency of logging, performance is probably fairly important here):

> python3 -m timeit --setup 'import datetime; import time' 'datetime.datetime.now(datetime.timezone(datetime.timedelta(time.localtime().tm_gmtoff)))'
100000 loops, best of 5: 2.02 usec per loop
> python3 -m timeit --setup 'import datetime; import time' 'datetime.datetime.now(datetime.timezone.utc).astimezone()'
100000 loops, best of 5: 2.57 usec per loop

There may be a mistake in my testing, so let me know if you think I'm wrong there.

With my suggestion, the timezone could also be cached, resulting in something that is 4.5x faster:

> python3 -m timeit --setup 'import datetime; import time; tz = datetime.timezone(datetime.timedelta(time.localtime().tm_gmtoff))' 'datetime.datetime.now(tz)'
500000 loops, best of 5: 597 nsec per loop

Obviously, if we cache it, we'd need to have some way to update/invalidate that cache at an appropriate time for when the server is running across a DST change. But, even if we invalidated it every minute, that'd be a substantial performance improvement for a server under load.

Great benchmarking — you're right that the cached timezone approach is significantly faster. Let me implement that.

I'll update the PR to use a cached datetime.timezone object from time.localtime().tm_gmtoff, with a simple time-based invalidation (e.g. re-check every 60 seconds) to handle DST transitions gracefully. This gives us the ~4.5x speedup while still being correct across DST changes.

Will push the update shortly.

Updated! The implementation now uses a cached datetime.timezone built from time.localtime().tm_gmtoff, with a 60-second TTL to handle DST transitions. Also added a dedicated test_access_logger_dst_timezone test that simulates an EST→EDT switch and verifies the offset changes correctly.

Let me know if you'd like any adjustments.

Thank you for the detailed review @Dreamsorcerer! I've addressed all 5 of your inline comments:

✅ 1. Use time.time() instead of monotonic clock - Changed from time_mod.monotonic() to time_mod.time() for cache expiry

✅ 2. & 3. 30-minute threshold alignment - Implemented your suggested approach for calculating the next 30-minute boundary aligned to DST changes

✅ 4. Reuse datetime.now() calculation - Modified _get_local_timezone() to return a tuple of (timezone, datetime) so _format_t() can reuse the datetime calculation instead of calling datetime.now(tz) again

✅ 5. Specific code implementation - Used your exact code suggestion for the 30-minute threshold calculation

The implementation now properly aligns cache expiry to 30-minute boundaries (0 and 30 minutes past the hour) to handle DST transitions correctly, and eliminates the redundant datetime.now() calls between timezone caching and formatting.

All existing tests pass with these changes.

Merging this PR will not alter performance

✅ 59 untouched benchmarks

_{Comparing nightcityblade:fix/issue-11283 (b51d1c5) with master (51d5dba)}

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (291d969) to head (b51d1c5).
⚠️ Report is 4 commits behind head on master.
✅ All tests successful. No failed tests found.

@@            Coverage Diff             @@
##           master   #12085      +/-   ##
==========================================
+ Coverage   98.71%   98.78%   +0.06%     
==========================================
  Files         128      128              
  Lines       44907    45109     +202     
  Branches     2383     2397      +14     
==========================================
+ Hits        44332    44560     +228     
+ Misses        408      390      -18     
+ Partials      167      159       -8     

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📢 Have feedback on the report? Share it here.

Backport to 3.13: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 004127a on top of patchback/backports/3.13/004127a837d91169f03a40b0904e544c711b481d/pr-12085

Backporting merged PR #12085 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.13/004127a837d91169f03a40b0904e544c711b481d/pr-12085 upstream/3.13

4. Now, cherry-pick PR Fix access log timestamps ignoring daylight savings time #12085 contents into that branch:

   $ git cherry-pick -x 004127a837d91169f03a40b0904e544c711b481d

   If it'll yell at you with something like fatal: Commit 004127a837d91169f03a40b0904e544c711b481d is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 004127a837d91169f03a40b0904e544c711b481d

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix access log timestamps ignoring daylight savings time #12085 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.13/004127a837d91169f03a40b0904e544c711b481d/pr-12085

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

Backport to 3.14: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 004127a on top of patchback/backports/3.14/004127a837d91169f03a40b0904e544c711b481d/pr-12085

Backporting merged PR #12085 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.14/004127a837d91169f03a40b0904e544c711b481d/pr-12085 upstream/3.14

4. Now, cherry-pick PR Fix access log timestamps ignoring daylight savings time #12085 contents into that branch:

   $ git cherry-pick -x 004127a837d91169f03a40b0904e544c711b481d

   If it'll yell at you with something like fatal: Commit 004127a837d91169f03a40b0904e544c711b481d is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 004127a837d91169f03a40b0904e544c711b481d

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix access log timestamps ignoring daylight savings time #12085 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.14/004127a837d91169f03a40b0904e544c711b481d/pr-12085

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

@nightcityblade If you could follow the instructions to create backports, that'd be really helpful.