On July 28, 2016, I opened an E-mail from Mozilla which was titled “Party like it’s 9,999”. I wondered if it was some odd item about Y2K, but it was not.
Dated the day before, it started “Tomorrow, July 28th, is the 10,000th day of the Web. We’re feeling pretty nostalgic about the past 10k days, so we’re going to be sharing some of our faves in Twitter tomorrow.”
The Worldwide Web started on March 12, 1989.
This made the 10,000th day of the Web, July 27, 2016. Mozilla was off by a day.
Off-by-one errors are one of the commonest types of programming errors. They can be surprisingly easy to make. As we see here.