05/03/2026 is 5 March in most of the world and 3 May in the US. When a bank CSV has no date above 12 in either position, nothing in the text says which. The AI Bookkeeper used to fall back to month first unless a header gave a hint.
A statement covers weeks, not months. Take 05/03, 06/03, 08/03, 09/03: read day first, that is 5 to 9 March. Read month first, it is May to September. The importer now measures the span both ways and, with at least four dated rows, takes day first when it fits in 45 days or less and the other reading takes 90 days or more. The file's result says the dates were inferred. A passed dayFirst still wins, and a file of two or three rows keeps the old default, because sparse US statements can legitimately span months.
The rule is a heuristic, tested on hand-made files only (ambiguous-dates.test.mjs); no real bank export was tried. Full suite: 654 pass.