Mab_813There's important context missing in this article. It wasn't Pliny the Younger himself who was "accused" of getting the date of Vesuvius' eruption wrong - after all, he was close and his uncle died.
However, we don't have his original letters. Like other texts from antiquity, it was preserved because it was copied over centuries.
And when people copy texts, they make mistakes. Often, editors of ancient texts have to work with several medieval manuscripts that differ in several ways and must then decide on a version they think is closest to the original text, which is then published.
In the case of this famous letter, the text edition said "nonum kal. septembres" (nine days before the Kalendae of September = August 24th) but when archaeological findings gave hints on a later date, classics researchers started to wonder whether someone in the Middle Ages had made a copying mistakes - after all, that mistake would only need 3-4 letters to be different (for instance, december instead of september). That's where this theory comes from. Researches have always assumed that Pliny himself knew the correct date, for obvious reasons.