use chrono::{DateTime, Utc}; pub trait SystemClock: Send + Sync {    /// Returns the current date and time in UTC.    fn utc_now(&self) -> DateTime<Utc>;} pub struct RealSystemClock; impl SystemClock for RealSystemClock {    fn utc_now(&self) -> DateTime<Utc> {        Utc::now()    }} #[cfg(any(test, feature = "test-support"))]pub struct FakeSystemClockState {    now: DateTime<Utc>,} #[cfg(any(test, feature = "test-support"))]pub struct FakeSystemClock {    // Use an unfair lock to ensure tests are deterministic.    state: parking_lot::Mutex<FakeSystemClockState>,} #[cfg(any(test, feature = "test-support"))]impl Default for FakeSystemClock {    fn default() -> Self {        Self::new(Utc::now())    }} #[cfg(any(test, feature = "test-support"))]impl FakeSystemClock {    pub fn new(now: DateTime<Utc>) -> Self {        let state = FakeSystemClockState { now };         Self {            state: parking_lot::Mutex::new(state),        }    }     pub fn set_now(&self, now: DateTime<Utc>) {        self.state.lock().now = now;    }     /// Advances the [`FakeSystemClock`] by the specified [`Duration`](chrono::Duration).    pub fn advance(&self, duration: chrono::Duration) {        self.state.lock().now += duration;    }} #[cfg(any(test, feature = "test-support"))]impl SystemClock for FakeSystemClock {    fn utc_now(&self) -> DateTime<Utc> {        self.state.lock().now    }}