Why the Gregorian Calendar Replaced the Julian Calendar (and Why Britain "Lost" 11 Days)
Every date on this site — tomorrow's date, this year's holidays, the monthly calendar — runs on the Gregorian calendar, the civil calendar now used almost everywhere in the world for business, government and daily life. It feels permanent and universal, but it's a relatively recent fix for a very old rounding error, and different countries adopted it years, decades or even centuries apart.
The problem: a year is not exactly 365.25 days
The Gregorian calendar's predecessor, the Julian calendar, was introduced by Julius Caesar in 46 BCE. It was a real improvement over what came before it, and its rule was simple: three 365-day years, then one 366-day leap year, giving an average year length of 365.25 days. The catch is that a true solar year — the time it actually takes Earth to orbit the Sun — is about 365.2422 days, roughly 11 minutes shorter. Eleven minutes a year sounds trivial, but it compounds: by the 1500s, the Julian calendar had drifted about 10 days ahead of the actual solar year.
That drift mattered most for one specific reason: the date of Easter, which the Catholic Church calculated relative to the spring equinox. As the calendar drifted, the equinox kept landing on an earlier and earlier calendar date than the rule assumed, slowly pulling Easter out of its intended season.
Pope Gregory XIII's fix, in 1582
In October 1582, Pope Gregory XIII introduced two changes. First, a one-time correction: Thursday, October 4, 1582 was followed directly by Friday, October 15, 1582 — ten days were simply removed from the calendar to re-align it with the equinox. Second, a refined leap-year rule to stop the same drift from happening again: a year is a leap year if it's divisible by 4, except century years, which are only leap years if they're also divisible by 400. That's why 2000 was a leap year but 1900 and 1700 were not — and it's the exact rule this site's own leap year guide and date engine still use today, over 440 years later.
Not everyone switched in 1582 — some waited centuries
Catholic countries — Spain, Portugal, Italy and Poland among them — adopted the new calendar almost immediately, in 1582. Protestant and Orthodox regions were far more reluctant, partly for religious reasons and partly because the drift hadn't reached 10 days when they'd have needed to adopt it themselves. The result was decades, in some cases centuries, where two dates for "today" were simultaneously in use across Europe.
Britain (and its American colonies) didn't switch until 1752 — by which point the gap had grown to 11 days. September 2, 1752 was followed directly by September 14, 1752. Contrary to a popular legend, there's little solid evidence of mass riots over "11 stolen days," but the switch was disorienting enough that landlords, employers and the Treasury all had to work out how to handle contracts and payments that spanned the gap. Russia held out even longer, only adopting the Gregorian calendar in 1918 after the revolution — which is why the historical "October Revolution" is now dated in November on the calendar most of the world uses.
It still isn't the only calendar in use
The Gregorian calendar is the near-universal civil calendar today, but it was never meant to replace every other calendar people use for religious and cultural purposes. The Islamic Hijri calendar, the Hindu lunisolar Panchang, the Hebrew calendar and several regional solar and lunisolar calendars are all still in active use for holidays and observances alongside it — which is exactly why some dates on our holiday pages are fixed years in advance while others depend on lunar observation and are marked "estimated" until confirmed. See our companion piece on how public holidays are actually decided, or the rundown of other calendar systems on our homepage, for more.