getDiffToOldAssets = (currentAssets, oldAssets) => {
	const diff = new Set();
	const now = Date.now();
	for (const [asset, ts] of oldAssets) {
		if (ts >= now) continue;
		if (!currentAssets.has(asset)) diff.add(asset);
	}
	return diff;
}
@param {Assets} currentAssets assets list
@param {Assets} oldAssets old assets list
@returns {Diff} diff