guarantee issues

Hi I bought a necklace at Game store on the 10th of November 2015 with a 3 months guarantee.The part of the necklace that keep the necklace together broke while on my neck.When I went to the store the manager, Brian, and sales lady told me the guarantee has expired on the 10th January.If I'm correct isn't it suppose to expire on the 10th February? The sale lady told me that they are counting from the 10 November till the 10th December as 2 months.Is this genuine to count from the day of purchase as one month of guarantee gone?