It looks like the example uses RFC 3339. The RSS spec specifies RFC 822, which is more human readable.
If someone is converting from an RSS feed or some pre-existing format, it might be onerous and error-prone to make them parse a datetime and then spit it back in RFC 3339. On the other hand, consumers of JSON-RSS would probably appreciate all timestamps being in a specific format.
Speaking of converting from RSS feeds, the standard there is to call the field "pubDate" not published_at (which seems more of a Rails-y name for a field).
It looks like the example uses RFC 3339. The RSS spec specifies RFC 822, which is more human readable.
If someone is converting from an RSS feed or some pre-existing format, it might be onerous and error-prone to make them parse a datetime and then spit it back in RFC 3339. On the other hand, consumers of JSON-RSS would probably appreciate all timestamps being in a specific format.
Speaking of converting from RSS feeds, the standard there is to call the field "pubDate" not published_at (which seems more of a Rails-y name for a field).