Copying a column with a bunch of dates in it, from one place to another, in LibreOffice, the free-ware replacement for Microsoft Excel.
Within the same spreadsheet, no problem, everything works just fine.
Copying from one spreadsheet to another, all the dates advance by four years and one day.
Once I realised it was a reliable pattern, and not just a random scrambling, it was easy enough to fix. But, WTF?