"""JSON-safe serialization helpers (contract: ISO 8601 UTC with `Z`)."""
from datetime import date, datetime, timezone


def to_iso(value: datetime | None) -> str | None:
    """Timestamptz -> '2026-08-02T10:00:00Z' (UTC). Naive values are treated as UTC."""
    if value is None:
        return None
    if value.tzinfo is None:
        value = value.replace(tzinfo=timezone.utc)
    return value.astimezone(timezone.utc).isoformat().replace("+00:00", "Z")


def to_date(value: date | None) -> str | None:
    """Date -> 'YYYY-MM-DD'."""
    return value.isoformat() if value is not None else None