Convert Openweathermap Sunrise JSON value to NSDate IOS

i'm working on a weather app for school and I don't understand at all how to read the sunrise value returned by the openweathermap API see below (we have to use JSON format) :

JSON :

  "sys":{  
     "sunrise":1418107981,
     "sunset":1418140433
  }

XML :

<sun rise="2014-12-09T06:52:59" set="2014-12-09T15:53:53"/>

the values are the same but in 2 different format ...

So my question is :

How can I convert this [ "sunrise":1418107981 ] into a NSDate object ?