Account payment

At the end of November I paid my account. Then on the 6th of December I paid my account again as that's when I got paid . In January I received a phone call telling me I did not pay my account in December . I told them I did pay and they said to me that because I paid on the 6th of December it still falls under November as their books close on the 7th . So now I have to make a double payment in January to cover the missed payment in December . This is really ridiculous as you think your doing a good thing paying your account early and now you get penalized with a double payment .