How do you know the year Harry Potter was born?

In the United States hard cover version of Harry Potter and the Deathly Hallows, on page number 328, it states the names of James and Lily Potter (Harry's Parents) and their date of birth and death. They died on the 31st of October, year 1981. They died a year after Harry was born. So Harry Potter was born in 1980. If we were an actual person and not a fictional character he would be 29 this year.