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JavaScript

var jsonDecoded = {
  "241019846088038":{
    "start_time":"20140815",
    "name":"Event 3"
    },
  "244875799004498":{
    "start_time":"20141229",
    "name":"Event 8"
    },
  "288519758006296":{
    "start_time":"20140809",
    "name":"Event 1"
    }      ,
  "576146455831402":{
    "start_time":"20140830",
    "name":"Event 14"
    }
  ,
  "1416493345292145":{
    "start_time":"20140920",
    "name":"Event 2"
    }
};

function compararDatas(a, b) {
        return a[1].start_time < b[1].start_time ? -1 :
               a[1].start_time > b[1].start_time ?  1 : 0;
}

var pares = _.pairs(jsonDecoded).sort(compararDatas);
var chaves = _.pluck(pares, 0);
var valores = _.pluck(pares, 1);

console.log(JSON.stringify(chaves));
console.log(JSON.stringify(valores));