Mary Beard talks about this in her book "Pompeii: The Life of a Roman Town". She points out that not only are there multiple versions of Pliny's letter with different dates, but also:
1) Many of the victims were wearing heavy woolen clothes. Hardly what you would expect on a day in late August in Italy.
2) Shops and restaurants have seasonal foods that are typical of late fall and early winter, not late summer.
3) The latest election ads throughout the town were for an election that took place well after late August
4) A coin was found in such a way that it could not possibly have been dropped by later looters. Currently believed that the earliest the coin could have been minted is late September of 79 (she doesn't give much detail about this).
So yes there is quite a bit of debate about when this happened.
1) Many of the victims were wearing heavy woolen clothes. Hardly what you would expect on a day in late August in Italy. 2) Shops and restaurants have seasonal foods that are typical of late fall and early winter, not late summer. 3) The latest election ads throughout the town were for an election that took place well after late August 4) A coin was found in such a way that it could not possibly have been dropped by later looters. Currently believed that the earliest the coin could have been minted is late September of 79 (she doesn't give much detail about this).
So yes there is quite a bit of debate about when this happened.