How do I convert a Unix timestamp string to time_t in C++11?

Short question, aka "TD;DR".

I have a, say, "1464478647000" string, that I guess is actually a UNIX timestamp. How do I convert it to a time_t type so I can format it later to a formated string, like "3 minutes ago" in C++11?

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Long question.

Sorry for my bad english, first of all.

I'm totally noob at C++11, and even C++; I just learnt a little of C and that was like 10 years ago, I almost forgot how it was. On my spare time, I'm trying to do a little fork of Feednix (a ncurses program, written in C++11, that acts as a Feedly client for Linux console - and looks like it's dead) so it looks more like a list (kind of how ncmpcpp or mutt look like). As the current Feednix implementation is not showing time of any post, I thought it would be nice to make it show the time it was published (as the web version of Feedly does, on its "Titles only" presentation).

The thing is that following the model of what's implemented on Feednix, I'm pulling the 'published' data as a string object (I couldn't figure how to pull it as an integer, or directly as a time_t object (seems the Json library doesn't allow to do that). Said 'published' data, says Feedly API docs, is "the timestamp, in ms, when this article was published, as reported by the RSS feed (often inaccurate)." An example of that is "1452614967000".

So how can I do to convert that string to a time_t object, so I can later format it to a string like "3 minutes ago" or "2 days ago"? Or is anything better I can do to get that formatted string (which is more likely)? Any help would be appreciated!