Add UTC datetime helpers to attempt to fix running issue

backend/app/tests/test_datetime_utc.py

@@ -0,0 +1,47 @@
+from datetime import datetime
+
+from app.core.datetime import unix_ms_utc, utc_now
+from app.schemas import EventCreate, EventResponse, EspTimeResponse
+
+
+def test_event_input_datetime_is_normalized_to_utc_naive() -> None:
+    event = EventCreate(
+        title="Evening briefing",
+        event_date="2026-06-01T19:30:00+01:00",
+        event_time=None,
+    )
+
+    assert event.event_date == datetime(2026, 6, 1, 18, 30)
+    assert event.event_date.tzinfo is None
+
+
+def test_response_datetimes_serialize_as_zulu() -> None:
+    event = EventResponse(
+        id=1,
+        title="Evening briefing",
+        description=None,
+        event_date=datetime(2026, 6, 1, 18, 30),
+        event_time=None,
+        location=None,
+        max_attendees=None,
+        status="draft",
+        created_by=1,
+        created_at=datetime(2026, 5, 1, 10, 0),
+        updated_at=datetime(2026, 5, 1, 10, 0),
+    )
+
+    payload = event.model_dump_json()
+
+    assert '"event_date":"2026-06-01T18:30:00Z"' in payload
+    assert '"created_at":"2026-05-01T10:00:00Z"' in payload
+
+
+def test_esp_time_uses_same_utc_instant_for_iso_and_unix_ms() -> None:
+    now = utc_now()
+    response = EspTimeResponse(
+        server_time_utc=now,
+        unix_ms=unix_ms_utc(now),
+    )
+
+    assert '"server_time_utc":"' in response.model_dump_json()
+    assert response.unix_ms == unix_ms_utc(now)