fix(atif): emit real UTC step timestamps instead of local time

src/nooa/atif/exporter.py

@@ -167,8 +167,16 @@ class _DispatchStep(NamedTuple):
 
 
 def _iso8601_utc(dt: datetime | None = None) -> str:
-    """ISO 8601 UTC timestamp (with millisecond precision and Z suffix)."""
-    dt = dt or datetime.now(UTC)
+    """ISO 8601 UTC timestamp (with millisecond precision and Z suffix).
+
+    ``dt`` is normalised to UTC first: the ``Z`` suffix asserts UTC, so
+    formatting a local-time value verbatim would mislabel it.  Event
+    timestamps arrive naive (local), while the default is already aware —
+    without the conversion a single trajectory mixes both, leaving steps
+    out of chronological order by the local UTC offset.  A naive value is
+    interpreted as local time, which is what the event system produces.
+    """
+    dt = datetime.now(UTC) if dt is None else dt.astimezone(UTC)
     return dt.strftime("%Y-%m-%dT%H:%M:%S.") + f"{dt.microsecond // 1000:03d}Z"
 
 

tests/atif/test_timestamp_timezone.py

@@ -0,0 +1,68 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+"""ATIF step timestamps must be real UTC, not local time wearing a Z suffix.
+
+``_iso8601_utc`` used to format whatever tzinfo it was handed and append ``Z``
+unconditionally.  Event timestamps arrive naive (local), while the no-argument
+default is already UTC-aware, so a single trajectory ended up mixing both and
+its steps were not in chronological order.
+"""
+
+import os
+import time
+from datetime import UTC, datetime, timedelta, timezone
+
+import pytest
+
+from nooa.atif.exporter import _iso8601_utc
+
+
+def _hour(stamp: str) -> int:
+    return int(stamp[11:13])
+
+
+class TestIso8601Utc:
+    def test_aware_utc_is_unchanged(self):
+        dt = datetime(2026, 7, 30, 10, 6, 42, 680_000, tzinfo=UTC)
+        assert _iso8601_utc(dt) == "2026-07-30T10:06:42.680Z"
+
+    def test_aware_non_utc_is_converted(self):
+        """A +02:00 wall clock of 12:06 is 10:06 UTC."""
+        dt = datetime(2026, 7, 30, 12, 6, 42, 680_000, tzinfo=timezone(timedelta(hours=2)))
+        assert _iso8601_utc(dt) == "2026-07-30T10:06:42.680Z"
+
+    def test_negative_offset_is_converted(self):
+        dt = datetime(2026, 7, 30, 6, 6, 42, 680_000, tzinfo=timezone(timedelta(hours=-4)))
+        assert _iso8601_utc(dt) == "2026-07-30T10:06:42.680Z"
+
+    def test_naive_is_interpreted_as_local(self):
+        """Naive event timestamps are local time and must be shifted to UTC."""
+        naive = datetime.now()
+        assert _iso8601_utc(naive) == _iso8601_utc(naive.astimezone(UTC))
+
+    def test_default_matches_explicit_now(self):
+        assert abs(
+            datetime.strptime(_iso8601_utc(), "%Y-%m-%dT%H:%M:%S.%fZ").replace(tzinfo=UTC)
+            - datetime.now(UTC)
+        ) < timedelta(seconds=5)
+
+    @pytest.mark.skipif(not hasattr(time, "tzset"), reason="tzset() is POSIX-only")
+    def test_naive_and_default_agree_under_a_non_utc_tz(self):
+        """The regression: two code paths must not disagree by the UTC offset.
+
+        Under a non-UTC local zone, an event-sourced (naive) timestamp and the
+        no-argument default previously landed hours apart in the same file.
+        """
+        prev = os.environ.get("TZ")
+        try:
+            os.environ["TZ"] = "Europe/Berlin"  # UTC+2 in July
+            time.tzset()
+            from_event = _iso8601_utc(datetime.now())  # naive → local
+            from_default = _iso8601_utc()  # aware → UTC
+            assert _hour(from_event) == _hour(from_default)
+        finally:
+            if prev is None:
+                os.environ.pop("TZ", None)
+            else:
+                os.environ["TZ"] = prev
+            time.tzset()