Confusing - is a 24 month contract in fact 25 months ?

I had a 24 month data contract - started 04/05/2017. I cancelled this contract in early April - giving the required one month calendar notice. I accepted that my debit order for early May would still be full - the contract bills monthly in arrears. I have just received my invoice for debit 01/06/2019 - still being billed for this data contract. I called the cancellations department and have been advised this is correct as "May is not finished" and data allocation was processed 01/05/2019. My question - if the contract started 04/05/2017 - 24 months expires 03/05/2019 - not 31/05/2019, that is 25 months. Please explain.