Even authors make mistakes about their characters. September 1 is the first day of Hogwarts for those in the “Harry Potter” stories. However, it’s not Albus Severus Potter’s first day yet — even though J.K. Rowling initially thought it was.

“If you’re at King’s Cross, the Potter, Granger-Weasley and Malfoy families are there too. Albus Severus starts school today. #19YearsLater,” Rowling said in a now-deleted tweet Thursday.

The problem is that Albus, Harry Potter’s middle child, hasn’t actually started school yet. The Battle of Hogwarts takes place May 2, 1998. Albus, as well as Rose Granger-Weasley and Scopius Malfoy, is said to start school 19 years after that date, which means he won’t take start Hogwarts until Sept. 1, 2017. His older brother, James Sirius Potter, started in 2015 while his younger sister, Lily Luna Potter, won’t have her first day at Hogwarts until 2019.

With over 8 million followers, Rowling was quickly informed of her mistake and deleted the message. She blamed her incorrect math on “Harry Potter and the Cursed Child,” the play that was released this year and follows Albus as he starts Hogwarts.

Rowling helped develop the story for the play along with director John Tiffany and writer Jack Thorne. Millions have bought the “Harry Potter and the Cursed Child” script, and tickets to the two-part show at London’s Palace Theatre are still hard to get. However, Rowling’s work isn’t over. She wants more people to see “Cursed Child,” especially those who helped keep spoilers under wraps.

“They’ve done me so proud,” Rowling said in an interview for the “Harry Potter and the Cursed Child” Twitter page. “I think our giveback is that we really hope to take this play to as many places as it’s feasible to take it. So I hope a lot of them will get to see this play in due course.”

If they’re lucky, New Yorkers might actually get to watch Albus, Scorpius and Rose head off to Hogwarts next year. The producers confirmed that they’re in the early stages of bringing “Cursed Child” to Broadway, but none of their plans are confirmed yet.