Ivar

Half converted and half text is the giveaway. A clean file fails consistently.

A check that catches this in one glance, for anyone who cannot change the import: after loading, count rows where the day part is thirteen or higher. If that count is zero on a full month of data, something has silently swapped and you are looking at the twelve-or-under half of the problem.

Real data almost always contains a day above twelve somewhere.