Leap Year Checker
Check if any Gregorian calendar year is a leap year. Unpack the astronomical 400-year solar cycle and see upcoming leap days.
2026 is NOT divisible by 4, so it is a common 365-day year.
2026 ÷ 4 = 506.50. Has remainder. Immediately disqualified.
2026 ÷ 100 = 20.26. Not a century year. Rule 1 confirmed leap year.
Not required because year is not a centurial milestone.
The Astronomical Reason for the 3-Tier Gregorian Leap Rule
The 365.2422 Solar Year Problem
Earth takes approximately 365 days, 5 hours, 48 minutes, and 45 seconds (365.24219 days) to orbit the sun. Adding a leap day every 4 years (0.25 days/year) slightly overcorrects by 11 minutes per year, accumulating a 3-day error every 400 years.
The 1582 Gregorian Reform (Pope Gregory XIII)
To rectify seasonal drift that had shifted the vernal equinox, the Gregorian reform decreed: Century years ending in "00" are NOT leap years unless they are also divisible by 400. Thus, 1600, 2000, and 2400 are leap years, but 1700, 1800, 1900, and 2100 are common years.
Famous Software Bugs on Leap Days
Leap day calculation bugs are notorious in software engineering. In 2008, all Microsoft Zune players froze globally on December 31 due to an infinite while-loop failing on day 366. In 2012, Microsoft Azure suffered a global outage caused by an unhandled leap year date calculation.