from datetime import UTC, datetime


def utc_now() -> datetime:
    return datetime.now(UTC)


def ensure_aware_utc(value: datetime) -> datetime:
    if value.tzinfo is None or value.utcoffset() is None:
        raise ValueError("naive datetime is not allowed")
    return value.astimezone(UTC)


def iso(value: datetime | None) -> str | None:
    if value is None:
        return None
    return ensure_aware_utc(value).isoformat().replace("+00:00", "Z")


def parse_iso(value: str | None) -> datetime | None:
    if value is None:
        return None
    parsed = datetime.fromisoformat(value)
    return ensure_aware_utc(parsed)