timeObservable.subscribe(new Observer<Long>() {

    long time = 0; // variable for demonstating how much time has passed

    @Override
    public void onSubscribe(Disposable d) {
        time = System.currentTimeMillis() / 1000;
    }
    @Override
    public void onNext(Long aLong) {
        Log.d(TAG, "onNext: " + ((System.currentTimeMillis() / 1000) - time) + " seconds have elapsed." );
    }
    @Override
    public void onError(Throwable e) {

    }
    @Override
    public void onComplete() {

    }
});