Hi, in SharePoint 2013 I have used a Search results web part to find newsfeeds. I have managed to do this okay but trying to sort the result in date order has been a real problem not matter what managed property I use. It can be descending or ascending and it does not make a blind bit of difference, as the result stays the same. I also notice that you get the same problem if you go on the Conversation search results page, trying to sort it by created or another managed property.
Setting sort property in search webpart is not enough. You need to create new result source file and set your sorting there and it will give you expected result. I have faced this issue and resolved!
Enjoy!