Leap years and day counts
Last updated: October 28, 2026
A day on the Days Since counter is 24 hours, but the calendar sometimes inserts an extra date. That extra date, February 29, makes some year-long spans 366 days and others 365. The counter does not have a leap-year switch. It uses the Gregorian calendar already built into the device, the same calendar that decides whether the day menu offers 29 days in February. This guide is how to predict the effect before you blame the total.
The rule that decides February 29
In the Gregorian calendar a year has a February 29 when the year number is divisible by 4, unless it is divisible by 100 but not by 400. 2024 is divisible by 4 and not by 100, so it is a leap year. 2026 is not divisible by 4, so it is not. 2000 is divisible by 400, so it is a leap year. 1900 is divisible by 100 and not by 400, so it is not, even though it is divisible by 4. The day menu follows that rule. February 2024 offers 29. February 2025 and February 2026 offer 28. February 1900, if you could pick it, would offer 28. The year menu on the homepage starts at 1960, so 1900 is outside the form, but the rule is the same one the browser uses for any date it can parse.
The counter does not invent a February 29 in a common year to be helpful. If you need “the last day of February,” pick February 28 in a common year and February 29 in a leap year. Those are different dates.
When the extra day lands inside the count
February 29 changes a count only when it sits strictly after the start instant and at or before the moment you are measuring. A start date is midnight at the beginning of that date, so February 29 as a start date does not count itself as an extra elapsed day. The next February 29 might.
From January 1, 2024 at midnight to January 1, 2025 at midnight is 366 days. February 29, 2024 is inside the span. From January 1, 2025 to January 1, 2026 is 365 days. There is no February 29 in 2025, and February 29, 2024 is already behind the start. From January 1, 2026 to January 1, 2027 is also 365 days. The calendar span of the homepage sample, January 1, 2026 to October 28, 2026, is 300 days partly because 2026’s February has 28 days and not 29. If 2026 were a leap year, that same pair of dates would be 301 days. It is not, and the calendar span is 300. Whether the counter shows 300 days and 0 hours at that midnight depends on daylight saving, which how calendar days are counted separates from the date difference.
A span that starts after February in a leap year waits until the next leap day. June 14, 2019 to June 14, 2026 includes February 29 of 2020 and of 2024, and excludes February 29 of 2028. Seven years at 365, plus those two leap days, is 2,557 days. That figure is the worked example in counting anniversary days. The anniversary is still the seventh anniversary. The leap days change the day count, not the count of June 14ths.
A date that exists only sometimes
Someone born on February 29, 2004 has a calendar birthday on February 29 in 2008, 2012, and every later leap year. In 2025 and 2026 there is no February 29 to select. The counter cannot observe a birthday the calendar does not contain. People who mark the birthday on February 28, or on March 1, in a common year are choosing a substitute. The page will count from the real February 29 you enter, and it will count toward whichever substitute you pick as an end date, but it will not tell you which substitute is correct. There is no general rule hiding in the script.
From February 29, 2024 at midnight to February 28, 2026 at midnight is 730 days. From February 29, 2024 to the next real February 29, which is February 29, 2028, is 1,461 days. Four common 365-day years would be 1,460; the extra day is February 29, 2028, the end date of that span. You can put February 29, 2024 into the counter today and read a live total. You cannot put February 29, 2026 in, because the day menu will not offer it, and a hand-edited URL for a date that does not exist is not a date the calendar agrees to.
365 is not a synonym for “one year later”
Adding 365 days to January 1, 2026 lands on January 1, 2027, because that span has no leap day. Adding 365 days to January 1, 2024 lands on December 31, 2024. The anniversary, January 1, 2025, is day 366 of that count. If you are watching the counter for “a year,” check whether February 29 falls inside it before you stop at 365. The milestone guide treats 365 as a round number of days, not as a promise about the month and day.
Century years are the cases people mis-remember. A span from March 1, 1899 to March 1, 1900 is 365 days, because 1900 is not a leap year under the 400-year exception, so there is no February 29 in that span. A span from March 1, 1999 to March 1, 2000 is 366 days, because February 29, 2000 falls after March 1, 1999 and before March 1, 2000. The 1900 case is the one that surprises people who only remember “every four years.” The homepage cannot select 1899 or 1900 — the menus begin at 1960 — so this paragraph states the rule completely. It is not an invitation to type those years into the form.
What the page will not smooth over
It will not average the year as 365.2425 days and show a fractional year. It will not call 365 days “close enough” to an anniversary. It will not move your February 28 entry to February 29 because a nearby year is a leap year. If the day menu and a paper calendar disagree about the length of February, trust the paper calendar and the Gregorian rule above, then write to contact@days-since.app with the year. A disagreement there would be a bug in the day menu, which asks the browser how many days the month has.