Neville Longbottom was born the same day as Harry Potter. Both parents of Neville (Frank and Alice Longbottom) were tortured to insanity by Bellatrix. When did this happen? How old was Neville when he had lost his parents?
When Harry first learns about the attack on the Longbottoms, Dumbledore explains that they were attacked after the murder of his parents, which took place on 31st October 1980:
“The Longbottoms were very popular,” said Dumbledore. “The attacks on them came after Voldemort’s fall from power, just when everyone thought they were safe.”
— Goblet of Fire, chapter 30, The Pensieve
We also see the trial of Karkaroff, presided over by Barty Crouch Sr., so this probably took place within a year or so of Voldemort’s downfall. Since it’s this trial where Crouch Jr. is accused of attacking the Longbottoms, this means the Longbottoms were attacked in late 1980 or 1981.
I don’t believe canon gives a date more precisely than that.
Being that the books clearly state that the attack on the Longbottom family occurred after Harry "vanquished" Voldemort the first time i.e. in October 31st 1981 this suggests that the Longbottoms were therefore attacked either in very late 1981 (between November and December) or early - mid 1982 (January to around June? Maybe July - August?) perhaps even as late as sometime in 1983 as it states that it was believed they were now safe, and rounding up as many of the Death Eaters as they must have to feel like that would have taken several months even with magic