JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = [{
    type: "green", 
    clusterId:42, 
    months:[1392185580000, 1394604780000, 1397279580000]
    }, {
    type: "red", 
    clusterId:43, 
    months:[1392185580000]
    }];

var store = {};

arr.forEach(function(d) {
    d.months.forEach(function(month) {
        if (!store.hasOwnProperty(month)) {
            store[month]=0;
        }
        store[month]++;
    });
});

console.log(store);