diff --git a/src/thoughtsync/auth.py b/src/thoughtsync/auth.py
index 520b083..80755ae 100644
--- a/src/thoughtsync/auth.py
+++ b/src/thoughtsync/auth.py
@@ -7,6 +7,7 @@ from datetime import datetime, timezone
 from quart import Blueprint, g, jsonify, request, session
 from sqlalchemy import func, select
 
+from .common import iso
 from .db import session_scope
 from .models.device_token import DeviceToken
 from .models.user import User
@@ -182,8 +183,8 @@ def _serialize_device(d: DeviceToken) -> dict:
     return {
         "id": str(d.id),
         "name": d.name,
-        "created_at": d.created_at.isoformat() if d.created_at else None,
-        "last_used_at": d.last_used_at.isoformat() if d.last_used_at else None,
+        "created_at": iso(d.created_at),
+        "last_used_at": iso(d.last_used_at),
     }