You are right, she made a big blooper there!
Here are my calcs:
Snape was Born January 9, c. 1958, attended Hogwarts School of Witchcraft and Wizardry from c. 1969 to c. 1976, was appointed Professor of Potions in c. 1981, and became Head of Slytherin house.
HBP takes place in year 1996. Snape would have been 38 years old then.
It would have been impossible for him to have been working at Hogwarts for 21 years cos it would mean that he would have been a professor at 17 years old. Hogwarts students only finish their schooling days at 17/18 years of age.
He started at Hogwarts when he was 23 years old. (1981 minus 1958 = 23 years).
That was a bad call on JK Rowling's part when she wrote that line.... 😆
Edited by Nishani - 19 years ago