Every context consumer re-renders

App re-renders often because of an unrelated timer. Profiling shows every component using AuthContext re-renders each time, even though user did not change. What is the cause and fix?

function App() {
  const [user, setUser] = useState(null)
  const [now, setNow] = useState(Date.now())
  // ... timer updates now every second
  return (
    <AuthContext.Provider value={{ user, setUser }}>
      <Routes />
    </AuthContext.Provider>
  )
}