J.K. Rowling revealed that Tuesday, September 1, 2015 was the day Harry and Ginny Potter's eldest child, James Sirius Potter, headed aboard the Hogwarts Express on his way to begin his magical education at Hogwarts School of Witchcraft and Wizardry.
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Harry Potter and the Deathly Hallows epilogue takes place two years from now, on September 1, 2017, meaning that Albus Potter would be two years younger than his older brother James. Harry and Ginny's youngest, Lily, as noted in the last chapter of the series, is two years younger than Albus.