fix(test): align baseline refresh slack and export timestamp rollover

Author: clean6378-max-it

Makefile

@@ -6,7 +6,7 @@
 seed-baselines-local:
 	@echo "WARNING: seed-baselines-local uses this host's timings; CI gates on ubuntu-latest." >&2
 	PYTHONPATH=. pytest tests/benchmarks/ --benchmark-only --benchmark-json=benchmarks/_raw.json -o addopts=
-	PYTHONPATH=. python scripts/reduce_baselines.py benchmarks/_raw.json benchmarks/baselines.json
+	PYTHONPATH=. python scripts/reduce_baselines.py benchmarks/_raw.json benchmarks/baselines.json --slack 1.5
 
 # Deprecated alias — kept for muscle memory; see seed-baselines-local warning above.
 update-baselines: seed-baselines-local

benchmarks/README.md

@@ -64,7 +64,7 @@ Or manually:
 
 ```bash
 PYTHONPATH=. pytest tests/benchmarks/ --benchmark-only --benchmark-json=benchmarks/_raw.json -o addopts=
-PYTHONPATH=. python scripts/reduce_baselines.py benchmarks/_raw.json benchmarks/baselines.json
+PYTHONPATH=. python scripts/reduce_baselines.py benchmarks/_raw.json benchmarks/baselines.json --slack 1.5
 ```
 
 Baselines must be captured on **ubuntu-latest** to match the gated CI runner. Cross-OS variance causes spurious failures.

tests/benchmarks/conftest.py

@@ -6,6 +6,7 @@
 import tracemalloc
 from collections.abc import Callable
 from copy import deepcopy
+from datetime import UTC, datetime, timedelta
 from pathlib import Path
 from typing import Any, TypeVar
 
@@ -18,6 +19,13 @@
 
 T = TypeVar("T")
 
+_EXPORT_SESSION_BASE = datetime(2026, 6, 12, 0, 0, tzinfo=UTC)
+
+
+def export_session_first_timestamp(index: int) -> str:
+    """Return a unique, valid ISO timestamp for export-corpus session *index*."""
+    return (_EXPORT_SESSION_BASE + timedelta(minutes=index)).strftime("%Y-%m-%dT%H:%M:%SZ")
+
 
 class TracemallocPeak:
     """Measure peak Python heap bytes for one callable invocation."""
@@ -102,7 +110,7 @@ def export_corpus(tmp_path: Path, request: pytest.FixtureRequest) -> Path:
     project.mkdir()
     for i in range(count):
         # Unique first_timestamp per session so export filenames do not collide in ZIP benches.
-        first_ts = f"2026-06-12T{i // 60:02d}:{i % 60:02d}:00Z"
+        first_ts = export_session_first_timestamp(i)
         write_jsonl(project / f"session_{i:04d}.jsonl", 20, first_timestamp=first_ts)
     return project