TimeStamper now returns UTC timezone for custom format string. (#713)

* TimeStamper now returns UTC timezone for custom format string.

* Add pull request link for #713

---------

Co-authored-by: Marcin Endraszka <marcin.endraszka.psnke@dralias.com>
Co-authored-by: Hynek Schlawack <hs@ox.cx>

Author: 3 people

CHANGELOG.md

@@ -15,6 +15,9 @@ You can find our backwards-compatibility policy [here](https://github.com/hynek/
 
 ## [Unreleased](https://github.com/hynek/structlog/compare/25.2.0...HEAD)
 
+### Fixed
+- `structlog.processors.TimeStamper` now returns UTC timezone for custom format string.
+  [#713](https://github.com/hynek/structlog/pull/713)
 
 ## [25.2.0](https://github.com/hynek/structlog/compare/25.1.0...25.2.0) - 2025-03-11
 

src/structlog/processors.py

@@ -553,12 +553,18 @@ def stamper_iso_utc(event_dict: EventDict) -> EventDict:
 
         return stamper_iso_local
 
-    def stamper_fmt(event_dict: EventDict) -> EventDict:
+    def stamper_fmt_local(event_dict: EventDict) -> EventDict:
         event_dict[key] = now().astimezone().strftime(fmt)
+        return event_dict
 
+    def stamper_fmt_utc(event_dict: EventDict) -> EventDict:
+        event_dict[key] = now().strftime(fmt)
         return event_dict
 
-    return stamper_fmt
+    if utc:
+        return stamper_fmt_utc
+
+    return stamper_fmt_local
 
 
 class MaybeTimeStamper:

tests/processors/test_renderers.py

@@ -414,6 +414,36 @@ def test_formats(self):
 
         assert "1980" == d["timestamp"]
 
+    @freeze_time("1980-03-25 16:00:00")
+    def test_inserts_formatted_utc(self):
+        """
+        The fmt string in UTC timezone works.
+
+        The exact hours calculated here maybe incorrect because of freezegun bugs:
+        https://github.com/spulec/freezegun/issues/348
+        https://github.com/spulec/freezegun/issues/494
+        """
+
+        ts = TimeStamper(fmt="%Y-%m-%d %H:%M:%S %Z")
+        d = ts(None, None, {})
+
+        assert "1980-03-25 16:00:00 UTC" == d["timestamp"]
+
+    @freeze_time("1980-03-25 16:00:00")
+    def test_inserts_formatted_local(self):
+        """
+        The fmt string in local timezone works.
+
+        The exact hours calculated here maybe incorrect because of freezegun bugs:
+        https://github.com/spulec/freezegun/issues/348
+        https://github.com/spulec/freezegun/issues/494
+        """
+        local_tz = datetime.datetime.now().astimezone().tzname()
+        ts = TimeStamper(fmt="%Y-%m-%d %H:%M:%S %Z", utc=False)
+        d = ts(None, None, {})
+
+        assert f"1980-03-25 16:00:00 {local_tz}" == d["timestamp"]
+
     @freeze_time("1980-03-25 16:00:00")
     def test_tz_aware(self):
         """