Why was this ever in doubt?
Pliny the Younger was across the Bay of Naples and witnessed the eruption from a distance.
Of interest, the two letters written by Pliny the Younger:
https://igppweb.ucsd.edu/~gabi/sio15/lectures/volcanoes/plin...
( hosted at the Institute of Geophysics and Planetary Physics as being the first recorded witnessing of a volcanic eruption )
The date is likely to be accurate as Pliny the Elder's will formerly adopted Pliny the Younger as a "son" and left him a considerable fortune.
The major eruption occurred in cycles over two days, subsequent ash falls that continued to kill may have persisted a week.
It's possible that 50 years on there may be some inaccuracy in the account wrt dates as Pliny the Younger may have referenced documents that dated when his uncles estate was settled rather than the date of his death.
As you say, this is a life-transforming event for Pliney the Younger. For him to get the date off by an entire season is quite a stretch.
But your question was why was his account of the date ever in doubt.
The answer is he wrote about it fifty years later in his 70's.
Did he have the date tattooed on his leg a day later? Did he get the date from an old newspaper he had lying about the house?
While he may be certain about his recollection of the date there is reasonable uncertainty about the certainty of a 70 year quoting a date from 5 decades past.
The question of the date currently has something like a one month window plus/minus .. so there's general agreement that it happened in the later part of 79 BC.
> For him to get the date off by an entire season is quite a stretch.
In my own experience it's not at all uncommon in the septuagenarian, octogenarian, and nonagenarian crowd to be off by that much in their stories.
The people who witnessed Pompeii in person probably remembers it pretty well. This is why I trust his recollection, even if Pliny wrote it down decades later.
It's also not unreasonable to assume he talked about it several times between the event and when it was written down, so that it was easy to remember the details.
It depends. In US is September eleven. In EU is November the nineth. /s
What actually survives are manuscripts which were copied from earlier manuscripts (iteratively). The ones we have are from the middle ages, usually copied by monks. It's very possible that a transcription error was made during this copying. In fact there's a whole field of study trying to second guess what the scribe might have miscopied, studying manuscripts, palimpsests and so on.
The British Museum in London has an excellent collection of manuscripts, with many now digitized: https://www.bl.uk/research/digitised-manuscripts/ (Unfortunately I can't seem to view the MS at this link, and they don't have Epistulae of Pliny the Younger which contains the two letters about Pompeii)