parse.js

var moment = require('moment');
var data = require('./one-week.json');

var t = {};

data.forEach(function(d) {
  var segments = d.segments;
  segments.forEach(function(s) {
    var activities = s.activities;
    activities.forEach(function(a) {
      if (a.activity == "wlk") {
        var timeStamp = moment.utc(a.startTime.replace(/[0-9]{4}Z/,''),"YYYYMMDDTHH").unix().toString();
        if (a.steps) {
          t[timeStamp] = (t[timeStamp]) ? t[timeStamp] + a.steps : a.steps; 
        }
      }
    });
  });
});

console.log(JSON.stringify(t));