The question is in red, Snape's response is in green, and my response is in purple. Marisol and Snape won't always both answer your question; you just get lucky sometimes.
Dear Marisol and Snape,
Why does Harry turn 21 in 2001? Doesn't he turn 15? Your math is badly flawed.
Sincerely, Susie Q
Harry was born in 1980, making your math flawed.
What Snape's not telling you is how we can tell Harry is born in 1980. If you look at the second book, go to the description of Nearly Headless Nick's deathday party. Now it was Nick's 500th deathday. He was killed in 1492. So, add 500 onto 1492 and you get 1992! Now, Harry is 12 in the second book, so subtract 12 from 1992 and you get 1980!! Subtract 1980 from 2001 and you get 21! Which is how old Harry is! Whoopie!!