Leap year problem
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The leap year problem (also known as the leap year bug or the leap day bug) is a problem for both digital (computer-related) and non-digital documentation and data storage situations which results from the wrong calculation of which years are leap years, or from manipulating dates without regard to the difference between leap years and common years.
Leap year bugs typically fall into two impact categories:
- Those that lead to error conditions, such as exceptions, error return codes, uninitialized variables, or endless loops
- Those that lead to incorrect data, such as off-by-one problems in range queries or aggregation
The following Python code is an example of a Category 1 leap year bug. It will work properly until
today becomes February 29. Then, it will attempt to create a February 29 of a common year, which does not exist. The
date constructor will raise a
ValueError with the message "day is out of range for month".
from datetime import date today = date.today() later = today.replace(year = today.year + 1)
The following Windows C++ code is an example of a Category 1 leap year bug. It will work properly until
st becomes February 29. Then, it will attempt to create a February 29 of a common year, which does not exist. Passing this to any function that accepts a
SYSTEMTIME struct will likely fail.
For example, the
SystemTimeToFileTime call shown here will return an error code. Since that return value is unchecked (which is extremely common), this will result in
ft being left uninitialized.
SYSTEMTIME st; FILETIME ft; GetSystemTime(&st); st.wYear++; SystemTimeToFileTime(&st, &ft);
The following .NET C# code is an example of a Category 1 leap year bug. It will work properly until
dt becomes February 29. Then, it will attempt to create a February 29 of a common year, which does not exist. The
DateTime constructor will throw an
DateTime dt = DateTime.Now; DateTime result = new DateTime(dt.Year + 1, dt.Month, dt.Day);
dt becomes February 29, such as on 2020-02-29. Then it will attempt to set the year to 2021. Since 2021-02-29 doesn't exist, the
Date object will roll forward to the next valid date, which is 2021-03-01.
var dt = new Date(); dt.setFullYear(dt.getFullYear() + 1);
Bad leap year algorithm (many languages)
The following code is an example of a leap year bug that is seen in many languages. It may cause either a Category 1 or Category 2 impact, depending on what the result is used for. It incorrectly assumes that a leap year occurs exactly every four years.
bool isLeapYear = year % 4 == 0;
The correct leap year algorithm is explained at Leap Year Algorithm.
There have been many occurrences of leap year bugs:
- In 2020 a large number of leap year bugs were cataloged in List of 2020 Leap Day Bugs at the website Code of Matt.
- In 2016, a leap year bug in the luggage conveyor system at Düsseldorf Airport on February 29 caused over 1,200 pieces of luggage to miss their flights.
- In 2016, a large number of leap year bugs were cataloged in List of 2016 Leap Day Bugs at the website Code of Matt.
- In 2012, Microsoft Azure was taken offline by the leap year bug on February 28. At 5:45 PM PST the Windows Azure team became aware of an issue, apparently due to a time calculation that was incorrect for the leap year.
- In 2012, Gmail's chat history showed a date of December 31, 1969, for all chats saved on February 29.
- In 2012, TomTom satellite navigation devices malfunctioned due to a leap year bug that first emerged on March 31.
- Sony's PlayStation 3 incorrectly treated 2010 as a leap year, so the non-existent February 29, 2010, was shown on March 1, 2010, and caused a program error.
- At midnight on December 31, 2008, many first generation Zune 30 models froze. Microsoft stated that the problem was caused by the internal clock driver written by Freescale and the way the device handles a leap year. It automatically fixed itself 24 hours later, but an intermediate "fix" for those who did not wish to wait was to drain the device's battery and then recharge after noon UTC on January 1, 2009.
- In 1996, two aluminum smelting plants at Tiwai Point, New Zealand, and Bell Bay, Tasmania, Australia, experienced a leap year bug on December 31, when each of the 660 computers controlling the smelting potlines shut down at the stroke of midnight simultaneously and without warning. The computers were not programmed to handle the 366th day of the year. Repair costs were estimated at more than NZ$1 million.
- Microsoft Excel has, since its earliest versions, incorrectly considered 1900 to be a leap year, and therefore that February 29 comes between February 28 and March 1 of that year. The bug originated from Lotus 1-2-3, and was purposely implemented in Excel for the purpose of backward compatibility. Microsoft has written an article about this bug, explaining the reasons for treating 1900 as a leap year. This bug has been promoted into a requirement in the Ecma Office Open XML (OOXML) specification.
- Johnson-Pint, Matt. "What are some examples of leap year bugs?". Stack Overflow. Retrieved 5 February 2020.
- Johnson-Pint, Matt. "Python - Replacing the year". Stack Overflow. Retrieved 29 February 2020.
- Johnson-Pint, Matt. "Win32 / C++ SYSTEMTIME struct manipulation". Stack Overflow. Retrieved 5 February 2020.
- Johnson-Pint, Matt. ".NET / C# - Construction from date parts". Stack Overflow. Retrieved 5 February 2020.
- Johnson-Pint, Matt. "Determining if a Year is a Leap Year". Stack Overflow. Retrieved 5 February 2020.
- Johnson-Pint, Matt. "List of 2020 Leap Day Bugs". Code of Matt. Retrieved 9 March 2020.
- "Airport hiccup leaves 100s of passengers pantless". The Local (de). Retrieved 5 February 2020.
- Johnson-Pint, Matt. "List of 2016 Leap Day Bugs". Code of Matt. Retrieved 5 February 2020.
- "TomTom sat-nav devices hit by GPS 'leap year bug'". BBC News. Retrieved 5 February 2020.
- "Sony fixes PS3 leap year bug". Metro. 2 March 2010. Retrieved 10 October 2019.
- "Home - Microsoft Answers". Forums.zune.net. Archived from the original on August 30, 2009. Retrieved 2011-07-27.
- John Herrman (2008-12-31). "30GB Zunes Failing Everywhere, All At Once". Gizmodo.com. Retrieved 2011-07-27.
- Geere, Duncan. "BREAKING: Zunes worldwide hit by mystery crash : Tech Digest". Techdigest.tv. Retrieved 2011-07-27.
- "Zune 30 FAQ". Microsoft. December 31, 2008. Retrieved January 1, 2009.
- Zadegan, Bryant (January 3, 2009). "A lesson on infinite loops". AeroXperience. Retrieved January 5, 2009.
- Towler, Jim. "Leap-Year software bug gives "Million-dollar glitch"". The RISKS Digest. ACM Committee on Computers and Public Policy. Retrieved 5 February 2020.
- Excel 2000 incorrectly assumes that the year 1900 is a leap year. Retrieved 2013-09-22.
- Standard ECMA-376 / Open Office XML File Formats. Retrieved 2016-09-10.
- ISO/IEC 29500 / Open Office XML File Formats. Retrieved 2016-09-10.