What are some examples of leap year bugs?

Question

A leap year bug is a code defect that creates a problematic, unintended outcome when executed within the context of a leap year, typically within the proleptic Gregorian calendar system.

The last leap year was 2016. The next leap years are 2020 and 2024.

There are two attributes that are unique to leap years:

• Leap years have a February 29th, while common years do not.
• Leap years have 366 days in total, while common years have only 365.

This post is intended to help others understand the nature of leap year bugs, what they look like in various languages, and how to correct them.

Leap year bugs typically fall into two impact categories:

• Category 1: Those that lead to error conditions, such as exceptions, error return codes, uninitialized variables, or endless loops
• Category 2: Those that lead to incorrect data, such as off-by-one problems in range queries or aggregation

With each answer, please indicate the programming language and/or platform, as well as the impact category as defined above. (Follow the template used by existing answers please.)

Please create one separate answer per language and type of defect, and vote for your favorite, especially those you have personally encountered (leaving comments with anecdotes where possible).

I will seed a few answers to get started, and update with additional examples over time.

Answer 1

.NET / C# - Construction from date parts

Impact Category 1

Defective Code

DateTime dt = DateTime.Now;
DateTime result = new DateTime(dt.Year + 1, dt.Month, dt.Day);

This code will work properly until dt becomes February 29th. Then, it will attempt to create a February 29th of a common year, which does not exist. The DateTime constructor will throw an ArgumentOutOfRangeException.

Variations include any form of DateTime or DateTimeOffset constructor that accepts year, month, and day parameters, when those values are derived from different sources or manipulated without regard to validity as a whole.

Corrected Code

DateTime dt = DateTime.Now;
DateTime result = dt.AddYears(1);

---

Common Variation - Birthdays (and other anniversaries)

One variation is when determining a user's current birthday without considering leaplings (persons born on February 29th). It also applies to other types of anniversaries, such as hire date, date of service, billing date, etc.

Defective Code

DateTime birthdayThisYear = new DateTime(DateTime.Now.Year, dob.Month, dob.Day);

This approach needs adjustment, such as the following which uses February 28th for common years. (Though, march 1st might be preferred depending on the use case.)

Corrected Code

int year = DateTime.Now.Year;
int month = dob.Month;
int day = dob.Day;
if (month == 2 && day == 29 && !DateTime.IsLeapYear(year))
    day--;

DateTime birthdayThisYear = new DateTime(year, month, day);

Corrected Code (alternative implementation)

DateTime birthdayThisYear = dob.AddYears(DateTime.Now.Year - dob.Year);

Answer 2

Win32 / C++ - SYSTEMTIME struct manipulation

Impact Category 1

Defective Code

SYSTEMTIME st;
FILETIME ft;

GetSystemTime(&st);
st.wYear++;

SystemTimeToFileTime(&st, &ft);

This code will work properly until st becomes February 29th. Then, it will attempt to create a February 29th 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.

Corrected Code

SYSTEMTIME st;
FILETIME ft;

GetSystemTime(&st);
st.wYear++;

bool isLeapYear = st.wYear % 4 == 0 && (st.wYear % 100 != 0 || st.wYear % 400 == 0);
st.wDay = st.wMonth == 2 && st.wDay == 29 && !isLeapYear ? 28 : st.wDay;

bool ok = SystemTimeToFileTime(&st, &ft);
if (!ok)
{
  // handle error
}

This fix checks for Feb 29th of a common year, and corrects it to Feb 28th.