hello.js

var hello = function() {
    return 'hello world';
};

var helloDate = function() {
    console.log(new Date());
    return 'hello world';
};

var returnDate = function() {
  var date = new Date();
  return date.toISOString();
};

test_hello.js

var assert = require('assert');
var sinon = require('sinon');
var vm = require('vm');
var fs = require('fs');

var source = fs.readFileSync('./hello.js').toString();

var tempContext = new vm.createContext({});
var script = new vm.Script(source);
script.runInContext(tempContext);

function createFunction(moduleName, funcName, funcContent) {
  return moduleName + '.' + funcName + ' = ' + funcContent;
}

var localModule = {};
for (var objName in tempContext) {
  var funcName = objName;
  var funcContent = tempContext[objName].toString();
  var toEval = createFunction('localModule', funcName, funcContent);
  eval(toEval);
}

var clock;

before(function() {
  var now = new Date('2017');
  clock = sinon.useFakeTimers(now.getTime());
});

it('hello', function() {
    assert.strictEqual(tempContext.hello(), 'hello world');
});

it('helloDate', function () {
    assert.strictEqual(localModule.helloDate(), 'hello world');
});

it('returnDate with fakeTimer', function() {
  assert.strictEqual(
    localModule.returnDate(), '2017-01-01T00:00:00.000Z'
  );
});