Discrepancy
The confession account contained a scribbled footnote from the elder Trevor that recorded the fatal note delivered to him. This contradicts the stated fact that Trevor never regained consciousness until the very end of his life, at which time he merely revealed where the confession lay.
According to Trevor's confession the Gloria Scott left Falmouth "thirty years ago" and precisely in 1855, but that would set Holmes's enquiry in 1885 and not in his college years, as he told Watson. Since further in the story the elder Trevor writes also: "For more than twenty years we have led peaceful and useful lives", we can assume that the exact date is around 1875, that would fit Holmes's reference to his college years. It is also possible that the "thirty years" was a rounding of twenty-five years, which would place the case in 1880.
Read more about this topic: The Adventure Of The Gloria Scott