Type the year 1900 into Microsoft Excel and it still treats it as a leap year. The error is deliberate, copied from Lotus 1-2-3 so that imported spreadsheets would not drift by a day. This episode traces how Mitch Kapor and Jonathan Sachs launched the program in 1983, outpacing a complacent VisiCalc with a spreadsheet, charting, and database package written in x86 assembly that wrote straight to the IBM PC’s video memory and refreshed almost instantly.

Lotus became so central that buyers judged clone computers by one question: can it run Lotus? By 1987, one in four of the 15 million Americans using a computer at work used it, and its macros locked whole businesses in. Then came the missteps: a slow, bloated rewrite in C called Release 3, a refusal to take Windows seriously, and a 1995 sale to IBM that drew punishing retaliation from Microsoft. The program survived for 31 years, and its leap year bug outlived it.

• Kapor spent an unheard-of $1 million on advertising in early 1983 and watched a focus group participant try to tear the plastic housing off a floppy disk, which led to manuals rewritten in plain English.
• Lotus 1-2-3 popularized the F1 key for context-sensitive help and let users run two video cards at once, with sharp text on one monitor and a live chart on the other.
• Its appetite for memory pushed past the 640 kilobyte DOS ceiling, leading Lotus to work with Intel and Microsoft on expanded memory standards.
• Lotus v. Borland ran six years over Quattro Pro’s copied menu hierarchy and ended in 1996 with a 4-4 Supreme Court tie after Justice John Paul Stevens recused himself.
• Microsoft withheld IBM’s Windows 95 license until 15 minutes before the retail launch, leaving IBM with nothing on shelves while Compaq and HP sold new machines.