Y2K leap year bug
n. A computer problem involving software that has been programmed incorrectly to treat the year 2000 as a non-leap year.
Examples
2000
Just when you thought it was safe to log on, here comes the dreaded Y2K Leap Year Bug. CERT — the respected Computer Emergency Response Team, based at Carnegie-Mellon University — has issued an advisory on how some computers will mistake Feb. 29 for March 1 .
—Janet Kornblum, “All-tech radio ready to launch,” USA Today, January 05, 2000
1998
And to add another cruel twist, a number of applications now in use have botched the dating scheme for leap year. Because the year 2000 is divisible by 400, there will be a Feb. 29 in the year 2000 even though there was not one in 1900. As a result, some programs with the Y2K leap year bug will assign erroneous dates every day after Feb. 28, 2000.
—Peter H. Lewis, “Exterminating the 2000 Bug From Home Systems,” The New York Times, July 21, 1998
1998 (earliest)
Cyber Cassandras flogging the Y2K (Year 2000) scare, the idea that the world will suffer a computer meltdown when the clock and calendar turns into the new millennium on Jan. 1, 2000, got something else to harp about last week. The Y2K Leap Year Bug raised its ugly little head. It turns out that lots of companies that fixed their computers so they won't think it's the year
1900 when the calendar turns (the Millennium Bug) will get tripped up anyway because they forgot that 2000 is a Leap year and 1900 was not.

The Federal Aviation Administration, for example, found that when its
computers are fed flight information for Feb. 29, 2000, they inform the user that there is no such date.
—James Coates, “Da mare on da net,” Chicago Tribune, February 16, 1998
Notes
This problem, a kind of baby brother bug of the much larger Y2K bug, was caused by programmers forgetting that there are actually three rules that determine whether a given year is a leap year:
  1. If the year is divisible by 4, then it's a leap year, unless…
  2. The year is also divisible by 100, then it's not a leap year, unless…
  3. The year is also divisible by 400, then it's a leap year.
Many programmers used only steps 1 and 2, and so concluded that 2000 was not a leap year.
Posted: February 10, 1999