from __future__ import annotations

from datetime import UTC, datetime


class Clock:
    """
    Abstraction over time for testability.
    """

    def now(self) -> datetime:
        """
        Current UTC datetime.
        """
        return datetime.now(UTC)


class FixedClock(Clock):
    """
    Fixed clock for deterministic testing.
    """

    def __init__(self, now: datetime):
        self._now = now

    def now(self) -> datetime:
        return self._now