diff --git a/src/thoughtsync/serialize.py b/src/thoughtsync/serialize.py
index 645dae5..e53b862 100644
--- a/src/thoughtsync/serialize.py
+++ b/src/thoughtsync/serialize.py
@@ -5,10 +5,22 @@ extras their surface needs (usage count in the labels API, sync_revision/purged_
 in sync deltas). Grows as sections adopt it (labels here; note/user/device later)."""
 from __future__ import annotations
 
+from .common import iso
 from .models.label import Label
 
 
 def serialize_label(label: Label) -> dict:
     """Base label shape {id, name, color}. The labels API adds `count`; sync deltas
-    (S4) add `sync_revision`/`purged_at`/`created_at` on top of this."""
+    add `sync_revision`/`purged_at`/`created_at` on top of this."""
     return {"id": str(label.id), "name": label.name, "color": label.color}
+
+
+def serialize_label_sync(label: Label) -> dict:
+    """Label as a sync delta row: the base shape + the fields native clients reconcile
+    on (monotonic revision, tombstone marker, creation time)."""
+    return {
+        **serialize_label(label),
+        "sync_revision": label.sync_revision,
+        "purged_at": iso(label.purged_at),
+        "created_at": iso(label.created_at),
+    }
diff --git a/src/thoughtsync/sync.py b/src/thoughtsync/sync.py
index a105d2d..d11de7b 100644
--- a/src/thoughtsync/sync.py
+++ b/src/thoughtsync/sync.py
@@ -19,6 +19,7 @@ from sqlalchemy import delete as sa_delete
 from sqlalchemy import func, select
 
 from .auth import login_required
+from .common import iso, parse_dt
 from .config import Config
 from .db import session_scope
 from .labeling import reconcile_manual_labels, resolve_owned_label_ids
@@ -37,6 +38,7 @@ from .notes import (
     normalize_color,
     normalize_recurrence,
 )
+from .serialize import serialize_label_sync
 
 bp = Blueprint("sync", __name__, url_prefix="/api/sync")
 
@@ -80,17 +82,6 @@ def _page_cursor(note_revs: list[int], label_revs: list[int], since: int, limit:
     return (max(all_revs) if all_revs else since), False
 
 
-def _serialize_label_row(lb: Label) -> dict:
-    return {
-        "id": str(lb.id),
-        "name": lb.name,
-        "color": lb.color,
-        "sync_revision": lb.sync_revision,
-        "purged_at": lb.purged_at.isoformat() if lb.purged_at else None,
-        "created_at": lb.created_at.isoformat() if lb.created_at else None,
-    }
-
-
 @bp.get("/changes")
 @login_required
 async def changes():
@@ -126,15 +117,18 @@ async def changes():
         note_rows = [n for n in note_rows if n.sync_revision <= cursor]
         label_rows = [lb for lb in label_rows if lb.sync_revision <= cursor]
 
+        # Note bodies come from the shared note serializer; sync adds the two
+        # delta-only fields on top (folding these into the serializer itself waits on
+        # the notes.py serialization split).
         notes_out = await _serialize_notes(db, note_rows)
         for data, n in zip(notes_out, note_rows):
             data["sync_revision"] = n.sync_revision
-            data["purged_at"] = n.purged_at.isoformat() if n.purged_at else None
+            data["purged_at"] = iso(n.purged_at)
 
         return jsonify(
             {
                 "notes": notes_out,
-                "labels": [_serialize_label_row(lb) for lb in label_rows],
+                "labels": [serialize_label_sync(lb) for lb in label_rows],
                 "cursor": cursor,
                 "has_more": has_more,
             }
@@ -144,15 +138,6 @@ async def changes():
 # --- Push: apply client mutations (LWW by client edit-time, non-destructive) ---
 
 
-def _parse_client_dt(raw: object) -> datetime | None:
-    if not isinstance(raw, str) or not raw:
-        return None
-    try:
-        return datetime.fromisoformat(raw.replace("Z", "+00:00"))
-    except ValueError:
-        return None
-
-
 def client_wins(client_edited_at: datetime | None, server_edited_at: datetime | None) -> bool:
     """Last-write-wins: the client's version is applied iff its edit-time is at least
     the server's. A missing client time never overwrites a real server edit; a missing
@@ -179,7 +164,7 @@ def _assign_note_fields(note: Note, ch: dict) -> None:
             note.deleted_at = datetime.now(timezone.utc)
     else:
         note.deleted_at = None
-    note.remind_at = _parse_client_dt(ch.get("remind_at"))
+    note.remind_at = parse_dt(ch.get("remind_at"))
     note.recurrence = normalize_recurrence(ch.get("recurrence"))
     if isinstance(ch.get("position"), int):
         note.position = ch["position"]
@@ -248,11 +233,15 @@ async def _apply_note(db, ch: dict) -> dict:
     except (ValueError, TypeError):
         return {"id": raw_id, "entity": "note", "status": "rejected", "error": "invalid id"}
     op = ch.get("op", "upsert")
-    edited_at = _parse_client_dt(ch.get("edited_at"))
+    edited_at = parse_dt(ch.get("edited_at"))
 
     note = await db.scalar(select(Note).where(Note.id == nid))
     if note is not None and note.owner_id != g.user_id:
-        return {"id": str(nid), "entity": "note", "status": "rejected", "error": "not yours"}
+        # A client only ever pushes ids of notes IT created, so this branch is only
+        # reached by a probe (or a ~0-probability UUID collision). Reject with a
+        # GENERIC message so the response doesn't confirm the id belongs to another
+        # user (don't leak existence via a distinctive "not yours").
+        return {"id": str(nid), "entity": "note", "status": "rejected", "error": "cannot apply"}
 
     if op == "delete":
         if note is None:
@@ -267,7 +256,7 @@ async def _apply_note(db, ch: dict) -> dict:
     creating = note is None
     if creating:
         note = Note(id=nid, owner_id=g.user_id, body="", display_title="")
-        created = _parse_client_dt(ch.get("created_at"))
+        created = parse_dt(ch.get("created_at"))
         if created is not None:
             note.created_at = created
         db.add(note)
@@ -309,11 +298,12 @@ async def _apply_label(db, ch: dict) -> dict:
     except (ValueError, TypeError):
         return {"id": raw_id, "entity": "label", "status": "rejected", "error": "invalid id"}
     op = ch.get("op", "upsert")
-    edited_at = _parse_client_dt(ch.get("edited_at"))
+    edited_at = parse_dt(ch.get("edited_at"))
 
     label = await db.scalar(select(Label).where(Label.id == lid))
     if label is not None and label.owner_id != g.user_id:
-        return {"id": str(lid), "entity": "label", "status": "rejected", "error": "not yours"}
+        # Generic rejection (see _apply_note): don't confirm a foreign-owned id exists.
+        return {"id": str(lid), "entity": "label", "status": "rejected", "error": "cannot apply"}
 
     if op == "delete":
         if label is None:
diff --git a/tests/test_sync.py b/tests/test_sync.py
index abf8153..2ce5b40 100644
--- a/tests/test_sync.py
+++ b/tests/test_sync.py
@@ -8,7 +8,6 @@ from thoughtsync.sync import (
     MAX_LIMIT,
     _clamp_limit,
     _page_cursor,
-    _parse_client_dt,
     _parse_since,
     client_wins,
 )
@@ -42,14 +41,6 @@ def test_client_wins():
     assert client_wins(None, None) is True
 
 
-def test_parse_client_dt():
-    assert _parse_client_dt("2026-07-22T00:00:00Z").year == 2026
-    assert _parse_client_dt("2026-07-22T00:00:00+00:00").tzinfo is not None
-    assert _parse_client_dt("garbage") is None
-    assert _parse_client_dt(None) is None
-    assert _parse_client_dt(123) is None
-
-
 def test_parse_since():
     assert _parse_since(None) == 0
     assert _parse_since("42") == 42