from datetime import datetime, timezone


def require_utc(value: datetime, field_name: str) -> datetime:
    if value.tzinfo is None:
        raise ValueError(f"{field_name} must include a timezone")
    return value.astimezone(timezone.utc)


def to_iso_z(value: datetime) -> str:
    return require_utc(value, "datetime").strftime("%Y-%m-%dT%H:%M:%SZ")