Microsoft Excel originally (and perhaps still?) treated the year 1900 as a leap year in its date computations even though it is not a leap year. It was an intentional choice. The existing spreadsheet software with the greatest market share at the time of Excel’s release was Lotus 1-2-3. Lotus 1-2-3 had the same behavior via a legitimate bug. By replicating the behavior, Excel could import Lotus 1-2-3’s file format with no unexpected outcomes for the user. This made the switch to Excel seamless and it quickly dominated the market.
Microsoft Excel originally (and perhaps still?) treated the year 1900 as a leap year in its date computations even though it is not a leap year. It was an intentional choice. The existing spreadsheet software with the greatest market share at the time of Excel’s release was Lotus 1-2-3. Lotus 1-2-3 had the same behavior via a legitimate bug. By replicating the behavior, Excel could import Lotus 1-2-3’s file format with no unexpected outcomes for the user. This made the switch to Excel seamless and it quickly dominated the market.
Nice note. In the 80s, I worked on a 3D spreadsheet program named BitsCalc (later BoeingCalc). We inserted the same behavior, for the same reason.
Fascinating!