"""Proposed implementation for this sample's explicit policy only."""
from datetime import datetime, timezone
from math import isfinite

def is_fresh(stored_at, now, ttl_seconds):
    # Policy: aware instants; finite nonnegative TTL; exact expiry is stale.
    for instant in (stored_at, now):
        if not isinstance(instant, datetime) or instant.utcoffset() is None:
            raise ValueError('Timezone-aware datetimes required')
    if isinstance(ttl_seconds, bool) or not isinstance(ttl_seconds, (int, float)):
        raise ValueError('Numeric finite nonnegative TTL required')
    if ttl_seconds < 0 or ttl_seconds > 3153600000 or not isfinite(ttl_seconds):
        raise ValueError('Numeric finite nonnegative TTL required')
    age = (now.astimezone(timezone.utc) - stored_at.astimezone(timezone.utc)).total_seconds()
    return 0 <= age < ttl_seconds