fn main() {
    rsx! {
        button {
            id: "start_stop",
            onclick: move |_| {
                timer
                    .with_mut(|t| {
                        t.started_at = if t.started_at.is_none() {
                            Some(Instant::now())
                        } else {
                            None
                        };
                    })
            },
        }
    }
}