Mocking a service with a promise, for testing within another service

I have the following service (reduced for simplicity):

angular.module('myApp')
    .factory('BasicTimerService', function BasicTimerService(PromiseService)
    {
        var isPlaying = false,
            maxEventTime = 0;

        return {
            start: function()
            {
                // only "start" once this promise has resolved.
                PromiseService.get_promise()
                    .then(function(value)
                    {
                        maxEventTime = PromiseService.maxEventTime();
                        isPlaying = true;
                    });
            },
            get_isPlaying: function() {
                return isPlaying;
            }
        };
    });

I wish to test that maxEventTime is only set after the Promise is successful.

My test:

describe('Service: BasicTimerService', function () {
    beforeEach(module('myApp'));

    var mockPromiseService,
        BasicTimerService,
        $q,
        $rootScope,
        deferred;

    beforeEach(function(){
        mockPromiseService = {
            maxEventTime: function() {
                return 17000;
            }
        };

        module(function($provide){
            $provide.value('PromiseService', mockPromiseService);
        });

        inject(function(_BasicTimerService_, _$q_, _$rootScope_){
            BasicTimerService = _BasicTimerService_;
            $rootScope = _$rootScope_;
            $q = _$q_;
        });

        mockPromiseService.get_promise = function() {
            var deferred = $q.defer();
            deferred.resolve('hello world');
            return deferred.promise;
        };
    });

    it('should get maxEventTime once the promise is resolved.', function() {
        spyOn(mockPromiseService, 'get_promise').and.callThrough();
        spyOn(mockPromiseService, 'maxEventTime').and.callThrough();

        $rootScope.$apply();

        BasicTimerService.start();

        expect(mockPromiseService.get_promise).toHaveBeenCalled();
        expect(BasicTimerService.get_isPlaying()).toBe(true);
        expect(mockPromiseService.maxEventTime).toHaveBeenCalled();
    });
});

I've determined that mockPromiseService.get_promise() is successfully called (the first expect() is successful), but the .then() clause in BasicTimerService does not seem to be called, and I cannot figure out why.

I have tried using both $rootScope.$apply(); as well as $rootScope.$digest(); to no avail.

Any ideas as to why this does not work?