Leap year rules and why February changes length
How the divisible-by-4, 100 and 400 rules decide February's length, why century years are special, and where the extra day comes from.
Why calendars need a leap day at all
Earth takes roughly 365.2422 days to orbit the Sun, not a neat 365. If the calendar ignored that extra quarter day, the seasons would slowly drift out of step with the months, and after a few centuries winter dates would land in summer. Adding one day to February every fourth year absorbs most of that fraction. February is the month that changes because, historically, it was the last month of the Roman calendar and the natural place to make an adjustment.
The divisible by 4, 100 and 400 rule
A simple every-fourth-year rule slightly overcorrects, because 0.2422 is a little less than a quarter. The Gregorian calendar fixes this by skipping the leap day in most century years: a year divisible by 100 is not a leap year unless it is also divisible by 400. That is why 1700, 1800 and 1900 were common years, while 1600 and 2000 were leap years. This tool encodes exactly that logic, so every February length it reports follows the official rule.
How accurate the Gregorian rule really is
Across a full 400-year cycle the calendar contains 97 leap years, giving an average year of 365.2425 days. That is only about 27 seconds longer than the true solar year, so the calendar stays aligned with the seasons for thousands of years before a single day of error builds up. For any practical planning, from contracts to schedules, the rule is effectively perfect. The next century year that will be a leap year is 2400.
Using month length in real planning
Knowing a month's exact length matters for payroll that pays a daily rate, for interest that accrues per day, and for anyone building a schedule that must line up with weekdays. Pairing the day count with the starting weekday tells you at a glance how the calendar grid will look. Because the tool also reports the weekday of the 1st, you can quickly work out the date of the first Monday or the last Friday without opening a full calendar.