The prophecy says, "Born as the seventh month dies." By the 30th, July is dying. It counts.
July is born from 1 July to 15 July. After that, it is nearing its end (dying), so anyone born between 15 and 31 July whose parents have opposed Lord Voldemort three times would be the Chosen One to kill Voldemort. The prophecy also might refer to both, as Neville Longbottom destroyed Nagini, Voldemort's last Horcrux, and was once the leader of Dumbledore's Army.