Example of how to use Sinon's useFakeTimers.

test-simple.js

/*global define, describe, it, before, after */
define(['chai', 'sinon', 'boilerplate/simple'], function (chai, sinon, simple) {
	'use strict';

	var assert = chai.assert;

	describe('simple', function () {
		describe('stamp', function () {
			var clock;

			before(function () {
				var date = new Date(2013, 3, 1);

				clock = sinon.useFakeTimers(date.getTime());
			});

			it('should append the date to the given value', function () {
				var actual = simple.stamp('test');

				assert.equal(actual, 'test:2013-04-01');
			});
			
			after(function () {
				clock.restore();
			});
		});
	});
});

To make this more resilient, your test should really be:

var actual = simple.stamp('test');
var expected = 'test:' + date.getFullYear() + '-' + ("0" + (date.getMonth() + 1)).slice(-2) + '-' + ("0" + date.getDate()).slice(-2);

assert.equal(actual, expected);