******.. They make you sign for 24 months but you'll pay for 25 months

I took a cellphone contact with vodacom in August 2017, I called to today ti give in a 30 days notice for them to cancel my contract at the end of this month as this is the 24th month. Only to be told that I can only cancel my contract at the end of August 2019.. I signed up for a 24 months contract, How does vodacom justify the 25th month as my first payment was in August 2017.. From August 2017 to August 2019, that is 25 months