private final long startNanos;
    private final Instant startInstant;

    /**
     * Private constructor to enforce singleton pattern.
     * Initializes the clock with the current system time and nanoTime.
     */
    private MonotonicClock() {
        this.startNanos = System.nanoTime();
        this.startInstant = Clock.systemUTC().instant();
    }

    /**

            MonotonicClock clock = MonotonicClock.get();
            Instant now = clock.instant();
            Duration elapsed = clock.elapsedTime();
            Duration calculated = Duration.between(clock.startInstant(), now);

            // Allow for small timing differences (1ms) due to execution time between measurements
            assertTrue(
                    Math.abs(elapsed.toMillis() - calculated.toMillis()) <= 1,