I would say that the tenancy periods run form 28th to 27th of each month and that to give valid notice, your Ts must inform you before 28th August that they intend to leave on/by 27th September, which would be the earliest they could end their tenancy. Had they wished it to end on 27th August they should have given notice before 28th July.
They can leave at the end of August if they wish, but the tenancy continues and they remain liable for rent until and including 27th September.
You may, if you wish, agree something different between you (if you are feeling generous!), but you are not obliged to.
I have dreamed in my life, dreams that have stayed with me ever after, and changed my ideas; they have gone through and through me, like wine through water, and altered the colour of my mind. Emily Bronte