The ancient spreadsheet package Lotus 1-2-3 set "0 January 1900" as its day zero. Whenever you entered a date into a Lotus spreadsheet, the program actually stored the number of days before or since that mythical date. Microsoft Excel needed to maintain compatability with Lotus early on, so it set 30 December 1899 as its day zero, which worked very well except for dates between 30 December 1899 and 1 January 1900, and it added the other mythical date 29 February 1900 because Lotus had that bug as well.
Flash forward a few decades, and today brings us a little bit of weirdness that only shows up in the American Month-Day-Year format.
Today is 4/21/15 in the U.S., and also day 42115 in Excel.
This bit of nerdiness brought to you by the Daily Water Cooler and one of The Daily Parker's readers.