My system short date format is set to `yyyy-mm-dd`.
I used Excel to create a spreadsheet, and included some dates, specifying `yyyy-mm-dd` formatting. When I looked at the resulting spreadsheet, I was surprised to see that Excel had stored the style not as `yyyy-mm-dd`, but rather as builtin style 14 (system short date format). Apparently the fact that the Excel styling matched my system choice was sufficient for it to override my choice! This is an astonishingly user-hostile implementation. Even though there are formats which, by design, "respond to changes in regional date and time settings", and even though the format I selected was not among those, Excel decided it was appropriate to vary the display even when I said I wanted an unvarying format.
This PR adds a new method `replaceBuiltinNumberFormat` to undo the damage that Excel does in such a situation. It also adds an `Excel Anomalies` document to the formal documentation, just to make situations like this readily available to the community.
BTW, Excel's sabotage can be avoided by using a number format style like `[Black]yyyy-mm-dd`.
oleibman