Section 21 Validity

Possession - Section 8 and Section 21

I asked solicitors to serve a section 21 notice on my tenants which they did by first class post. They also served by email and by hand on 15th April 2021. The tenancy agreement commenced on 14th December 2020 for a fixed term of 6 months. The section 21 notice and covering letter were both dated 13th April 2021 but were not deemed served until 15th April 2021. A second solicitor has now informed me that there is a risk that the notice will not be held to be valid because it is dated 13th April 2021 which is less than four months from the commencement of the tenancy.
Please can you advise whether the notice date is the relevant factor or whether the court will simply work on the date of service.