All the talk of brood cycles is in reference to specific things. The reality of it is once a hive is 21 days old bees emerge daily. There are instances when the queen slows production. However, during summer season she lays every day. so the eggs laid today will emerge as worker 21 days from today. Those laid tomorrow will emerge in 22 days from today. The bees within a hive are of varying ages thus a given hive has bees available to perform all tasks. Even within a hive that suffers a catastrophe causing a severe loos of house age bees. Foragers will regress to house bees to fill the void. So you have bees taking orientation flights daily. It is not like a shift change at the factory.