How leap years affect date calculations

The leap-year rule

A Gregorian calendar year is a leap year when:

• the year is divisible by 4;
• unless it is also divisible by 100;
• but a year divisible by 400 is a leap year after all.

The century exception is why 2000 contained February 29 but 1900 did not. It also means 2100 will not be a leap year, even though a simple every-four-years pattern might suggest otherwise.

How the extra day changes a date difference

A period crossing February can be one day longer when February 29 sits inside the range. For example, 1 January 2028 to 1 January 2029 spans 366 calendar days. The same dates across most neighbouring years span 365 days.

The extra day matters to a total expressed in days, but it does not mean every anniversary is delayed. A fixed annual event such as Christmas still lands on December 25. The number of days between January and that event is simply one day larger when leap day is inside the interval.

Adding one year to February 29

February 29 does not exist in a normal year, so software needs an explicit fallback rule. DaysUntil clamps an annual adjustment to the last valid day of the destination month. Adding one year to 29 February 2028 therefore produces 28 February 2029 rather than rolling into March.

The same principle applies at month ends. Adding one month to a date such as 31 January uses the final valid day in February. This preserves the idea of a month-based adjustment better than allowing the result to spill unpredictably into the following month.

Birthdays and anniversaries

People born on February 29 still become a year older each year, but the date used for a celebration or a legal age rule can vary by personal choice and local law. A calculator can apply a consistent calendar fallback, but it should not be treated as a legal ruling for eligibility, contracts or official deadlines.

Business-day calculations

Leap day is handled like any other calendar date. It counts as a potential business day when it falls Monday to Friday, unless a relevant holiday rule removes it. The additional date can therefore increase both a calendar-day count and a business-day count, but not necessarily by the same amount.

Check a date range

• Use days between dates to see whether February 29 adds a day to a particular range.
• Use add or subtract a date to test a month-end or leap-day adjustment.
• Read calendar days vs business days when weekends and public holidays also affect the answer.