import time


class MonotonicClock:
    """Single monotonic reference for latency/timeout math (1879 §2).

    Wall-clock (`now_wall`) is only for human-facing timestamps; all elapsed/
    timeout math must use `elapsed()` so NTP re-sync can't produce negatives.
    """

    def __init__(self):
        self._start = time.monotonic()

    def elapsed(self) -> float:
        return time.monotonic() - self._start

    @staticmethod
    def now_wall() -> float:
        return time.time()