Today I learned that Excel incorrectly assumes 1900 was a leap year, and Microsoft will never fix it because the whole house of cards would collapse

Excel contains a long-standing, known bug that incorrectly assumes the year 1900 was a leap year, a problem inherited from the influential spreadsheet program Lotus 1-2-3. When Microsoft developed its own spreadsheet software, Multiplan and later Excel, it prioritized compatibility with Lotus 1-2-3's serial date system, thus adopting the same historical error. This allowed for easier data migration and user adoption, but created a permanent inaccuracy in date calculations for that specific year. Despite acknowledging the bug, Microsoft has stated it will not fix it because correcting the issue would shift nearly all existing dates in current Excel worksheets by one day, causing significant disruption and incompatibility. The company concludes that the disadvantages of such a fix, including the immense time and effort required, far outweigh any potential advantages.