> Indeed, as you work on your Excel clone, you’ll discover all kinds of subtle details about date handling. When does Excel convert numbers to dates? How does the formatting work? Why is 1/31 interpreted as January 31 of this year, while 1/50 is interpreted as January 1st, 1950? All of these subtle bits of behavior cannot be fully documented without writing a document that has the same amount of information as the Excel source code.

A quick note to anybody building an Excel clone: If you want to turn this insane date handling behavior of Excel into an optional feature that can be disabled everybody will appreciate it.