The expiry date of a section 21 a (periodic tenancy).

Possession - Section 8 and Section 21

We have issued a number of Section 21's, fixed and periodic tenancies and used these to get possession through the court, same court at Oldham. We have just has one returned that the judge has ordered to be struck out because he says the expiry date of the notice is not on a rent date. The tenancy started over 12 months ago, no new agreement has been issued since they moved in so its a periodic tenancy. The notice was issued over 2 1/2 months prior to this and the notice states that the section 21 will expire on the last day of the tenancy, they moved in on the 10th of the month, the notice expired on the 9th, this being the last day of the tenancy, the notice states it expires on or after the 9th. I have looked on the internet, and the housing act wording and the notice we have issued appears to be correct, has anyone got a definitive answer - Also is a Section 21 b the same, or does this need to expire on a rent date? we have never had a section 21 dismissed before because of this and we have always done them the same way, i think it just must be this certain judge.