Add UTC datetime helpers to attempt to fix running issue

backend/app/core/datetime.py

@@ -0,0 +1,30 @@
+from datetime import datetime, timezone
+
+
+def utc_now() -> datetime:
+    """Naive UTC datetime for existing SQLAlchemy DateTime columns."""
+    return datetime.now(timezone.utc).replace(tzinfo=None)
+
+
+def to_utc_naive(value: datetime | None) -> datetime | None:
+    if value is None:
+        return None
+    if value.tzinfo is None:
+        return value
+    return value.astimezone(timezone.utc).replace(tzinfo=None)
+
+
+def to_utc_aware(value: datetime | None) -> datetime | None:
+    if value is None:
+        return None
+    if value.tzinfo is None:
+        return value.replace(tzinfo=timezone.utc)
+    return value.astimezone(timezone.utc)
+
+
+def to_zulu_iso(value: datetime) -> str:
+    return to_utc_aware(value).isoformat().replace("+00:00", "Z")
+
+
+def unix_ms_utc(value: datetime) -> int:
+    return int(to_utc_aware(value).timestamp() * 1000)