Gregorian calendar

The Gregorian calendar is the civil calendar most of the world runs on: a 1582 refinement of the Julian calendar that corrected its leap-year rule so the seasons stop drifting against the date. It sets the ordinary year at 365 days, adds a leap day in years like 2024 to make 366, and adjusts which century years qualify as leap years so the long-run average lands at 365.2425 days.

What changed

The Julian calendar, fixed by Julius Caesar’s 45 BC reform, gave every fourth year 366 days with no exception, for an average year of exactly 365.25 days. The problem: a year is about 11 minutes shorter than that, so the Julian calendar drifted roughly one day every 128 years. Pope Gregory XIII’s 1582 reform closed the gap with one extra rule: century years skip the leap day unless they’re divisible by 400. So 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. Spread over a 400-year cycle, that rule produces exactly 97 leap days instead of 100, pulling the average year down to 365.2425 days — close enough to the true solar year that the drift becomes negligible on any timescale that matters to a reader alive today.

Where you feel it

The clearest trace of the system is 29 February itself, the day that exists only because a year is not a tidy multiple of 365 days. The other trace is historical: the record shows that October 1582 skipped ten days outright in the first adopting countries, the 4th being followed by the 15th, to erase the drift the Julian calendar had already built up. Adoption spread over centuries rather than happening at once — Britain and its colonies switched only in 1752, by which point they had to skip eleven days. Historians date any document from before a country’s switch as “old style,” which is why archives sometimes show two dates for the same event depending on which calendar the writer was using. The Roman calendar’s own history, as the record has it, explains some of the naming oddities still in use: it originally began the year in March, so February, the last month in that scheme, absorbed whatever days were left over, and the months now called September to December carry Latin names meaning seven through ten because they were once the seventh through tenth months rather than the ninth through twelfth.