Is My S21 Ok?

Hi All,

Sorry I couldn't work out how to attach the S21, so please consider:

I recently served a S21 as follows:
Signed and served: 16-05-13
Wording: ...possession is required (Under Section 21...), of the premises..., at which you are a tenant, ON THE: 17-07-13, which shall be...

The tenancy began on the 17-03-12.

Are these dates ok? (as I've used wording 'on the' rather than 'after')
I would like to use it for eviction process but am worried as I've noticed other S21's using wording that says requires possession AFTER the end of the tenancy period.

Thanks v much,
Ad