15th is probably right. But?
If you let them go, you have a chance of one hive making it. If you split them, you have a chance of two hives making it, unless they aren?t strong enough to split. How long do you think you have left in your area of them laying brood. That would be my reference if looking for an answer. But I?m a gambler. It?s an inconsequential thing to me. Bees are easy to make if you have a enough for stock. It?s a fun thing for me, not my lively hood.