The publishedAt getting returned from a Playlist query is the date the video was added to the playlist, rather than the date the video was published on YouTube.
For example, the first two items in the playlist say:
"publishedAt": "2015-03-14T14:39:51.000Z"
"publishedAt": "2015-03-14T14:40:20.000Z"
You may need to loop through the results and do another request (can use multiple URL-encoded, comma-separated ids to save requests) on each videoId to get the actual published dates
GET https://www.googleapis.com/youtube/v3/videos?part=snippet&id=etAIpkdhU9Q%2CLo2qQmj0_h4&key={YOUR_API_KEY}
"publishedAt": "2013-03-08T11:15:44.000Z"
"publishedAt": "2013-03-11T07:00:41.000Z"