On the night of Wednesday, September 2, 1752, people across Britain and its American colonies went to bed as usual. The next morning they woke up on Thursday, September 14.

Nobody had overslept. Parliament had simply deleted eleven days. To understand why, we need to go back almost 1,800 years, to a calendar that was wrong by about eleven minutes.

Julius Caesar's tiny mistake

In 46 BC, Julius Caesar introduced a new calendar for Rome. It was a big improvement on what came before: 365 days a year, plus one extra day every fourth year. That makes the average year 365.25 days long. Neat and simple.

The trouble is that the real solar year, the time it takes for the seasons to come round again, is a little shorter: roughly 365.2422 days. The Julian calendar was too long by about 11 minutes a year.

Eleven minutes doesn't sound like much. But it adds up to about one full day every 128 years. Over many centuries, the calendar slowly drifted away from the sun, and dates like the spring equinox crept earlier and earlier on the calendar.

The Pope fixes it (for some people)

By the 1500s the drift was around ten days, and it was causing real headaches for working out the date of Easter, which depends on the spring equinox. In 1582 Pope Gregory XIII introduced a reformed calendar. It kept the leap year every four years, but skipped it in century years unless they divide by 400. So 1700, 1800 and 1900 are not leap years, but 2000 was.

To catch up with the sun, the reform also removed ten days in one go. In countries that switched immediately, Thursday, October 4, 1582 was followed by Friday, October 15.

But this was the 16th century, and Europe was deeply divided. Many Catholic countries adopted the new "Gregorian" calendar quickly. Protestant Britain was in no hurry to take orders from Rome, and stuck with the old one.

A century and a half of date confusion

For about 170 years, Britain and much of continental Europe were living on different dates. A letter written in London and one written in Paris on "the same day" could carry dates more than a week apart. Merchants and diplomats sometimes wrote both dates to avoid mix-ups.

To make things even more fun, England officially started its year not on January 1 but on March 25. So a date in February could belong to one year in London and the next year elsewhere. Historians still write dates like "February 1731/32" because of this.

1752: Britain finally gives in

By the 1750s the gap had grown to eleven days, and Parliament passed the Calendar Act to bring Britain and its colonies into line. Two big changes happened:

  • The year would officially begin on January 1, starting with 1752.
  • Eleven days would be dropped: September 2, 1752 would be followed by September 14.

The weekdays carried on without a break, Wednesday then Thursday, so the weekly rhythm of markets and church services wasn't disturbed. Lawmakers also thought about money: the Act made sure rents, wages and debts fell due after the full real time had passed, so nobody was cheated out of eleven days of pay or charged for days that never happened.

"Give us our eleven days!" (or did they?)

You might have read that angry crowds rioted in the streets, chanting for their stolen days back. It's a great story, and it gets repeated a lot. But historians have found little solid evidence for real calendar riots. Much of the legend seems to trace back to political arguments of the time and to a famous satirical painting by William Hogarth, which shows a banner reading "Give us our Eleven Days".

There certainly was grumbling and confusion, though. Imagine trying to work out a birthday, an anniversary or a contract date across that gap.

George Washington's two birthdays

One of the most famous people affected was George Washington. He was born in Virginia on what was then recorded as February 11, 1731. After the switch, the eleven extra days, plus the move of New Year to January 1, turned that into February 22, 1732, which is the date the United States celebrates today.

Why your tax year is still weird

The strangest leftover is in the United Kingdom. Before 1752, a key date for paying taxes and rents was Lady Day, March 25, the old start of the year. Rather than lose eleven days of tax, the treasury effectively moved the end of the tax year forward eleven days, to April 5. A later adjustment nudged it once more, and to this day the UK tax year starts on April 6. A bureaucratic fossil from 1752, still alive in every British payslip.

The last stragglers

Britain was far from the last to switch. Sweden had a famously messy attempt that briefly gave it a February 30 in 1712. Russia didn't change its civil calendar until 1918, which is why the "October Revolution" of 1917 actually happened in November by most of the world's calendars. Greece switched in 1923.

The calendar still isn't perfect

Even the Gregorian calendar is off by a few seconds a year compared with the seasons. It's so small that it will take thousands of years to add up to a single day. For now, your calendar, your phone and the clock on clocbox all agree, and that agreement took humanity about two thousand years of arguing to reach.