In any case, that's the story of why October 4th 1582 was followed by October 15th.

That days can be removed from (or added to) the calendar is a wonderful reminder of our strange relationship with time, of how some things which seem permanent — like the date — really aren't.

Other countries were even slower — in Russia the Gregorian Calendar wasn't adopted until 1918.

Hence the famous confusion around the "October Revolution" of 1917.

It happened on 25th October in the Julian Calendar, but on 7th November in the Gregorian.

By the 18th century, however, it had become clear that the Gregorian Calendar's accuracy made it better than the Julian Calendar.

Prussia was the first Protestant state to switch over, followed by Denmark-Norway, the Protestant Netherlands, Switzerland, and others.

But Europe was split religiously — both Protestant and Orthodox countries saw Gregory's new calendar as fundamentally Catholic and therefore resisted it.

And so, for a time, crossing the border from one country to another could also involve going forward or backward 10 days.

And in 1582 Gregory issued a decree called "Inter gravissimas".

It stated that the new, modified version of the Julian Calendar was to be adopted that year.

The change would happen in October, when Thursday 4th would be followed by Friday 15th, to correct the 10 days of drift.

The calendar also needed resetting.

And, crucially, Gregory reset it to the time of the Council of Nicaea in 325 AD, when the method for dating Easter was established — rather than to the birth of Christ or creation of the Julian Calendar.

So 10 days, not 14, had to be removed.

Scholars and clerics had been agitating for reform for decades — even during the Dark Ages people had noticed that the Julian Calendar meant Easter wasn't being observed at the right time.

So, in 1582, Pope Gregory XIII finally decided to solve this problem.

But during the wars and civil strife of the 1st century BC these intercalary months had not been added properly, and the old calendar had fallen badly out of sync with the solar year.

Caesar consulted Rome's best astronomers and came up with a solution — the Julian Calendar.

And it begins with Julius Caesar.

The year was 46 BC and he had just become "Dictator for Life" — Caesar was the sole ruler of Rome.

Among the many problems he needed to solve was the Roman Calendar, which had fallen into total chaos.

In the year 1582 something strange happened.

Thursday 4th October was followed immediately by Friday 15th October.

This is the story of history's 10 missing days...