The issue is due to the fact that MobileSheets strips punctuation while sorting, so the numbers become much larger numbers, i.e. 20231217 compared to 20241220, so it's sorting based on those numbers. If you placed a space between them, that would work without issue. Or if you make sure to always use two digits for the day and month as I suggested above (2024.01.09) that would also ensure that the numbers are always processed correctly. The other benefit of doing that is that the width of the dates will always be consistent, potentially making it more visually pleasing.
Mike
Mike