The Unix Billennium is the point in time represented by a Unix time value of 10⁹: 01:46:40 UTC on September 9 2001. Some programs which stored timestamps using a text representation encountered sorting errors, as in a text sort times after the turnover, starting with a "1" digit, erroneously sorted before earlier times starting with a "9" digit. Affected programs included the popular usenet reader KNode and email client KMail, part of the KDE desktop environment. Such bugs were generally cosmetic in nature and quickly fixed once problems became apparent.

The Unix Billennium is sometimes described as "10⁹ seconds after the Unix epoch". This is not quite correct, because Unix time is not a purely linear count of seconds. "10⁹ non-leap seconds after the Unix epoch" is closer to being accurate, but the epoch is in 1970 and leap seconds weren't actually defined before 1972.

The name is an amalgamation of "billion" and "millennium", recalling the year 2000 bug. The name is not very logical as billennium should rather mean a billion years. Billesecum might have been a better term.

The word "billennium" has also been used, presumably as abbreviated form of bi-millennium, for the year 2000 celebrations, by the Billennium Organizing Committee (BOC), who claims to own the term as registered trademark.

The word "billennium" is also used by such writers as Madeleine L'Engle (A Wind in the Door ) and Stanislaw Lem (Imaginary Magnitude), apparently in reference to a billion years.

The word "billennium" should not be confused with "biennium", a period of two years.

External links

• UNIX Approaches its One Billion Second Milestone