The problem was Julius Caesar’s leap year. The Julian calendar’s year of 365.25 days overshoots the actual solar year by about 11 minutes — trivial annually, but compounding to a full day every 128 years. By the 16th century the spring equinox, pegged by the Church to 21 March for calculating Easter, was actually falling on the 11th: Christendom’s most important feast was drifting toward summer on an error of arithmetic.
Pope Gregory XIII’s commission — with the Jesuit mathematician Christopher Clavius as its engine, building on the proposals of the deceased physician Aloysius Lilius — produced the two-part fix the world still runs on: skip leap years in century years not divisible by 400 (so 1700, 1800, 1900 lose theirs; 2000 kept it), and delete the accumulated error outright. The bull Inter gravissimas ordered that Thursday 4 October 1582 be followed by Friday 15 October. Ten days ceased to exist. St Teresa of Ávila obligingly died in the night between them, giving hagiographers a date puzzle at the outset.
Two Europes, two dates
Catholic states — Spain, Portugal, Poland, most of Italy, soon France — complied within months. Protestant Europe, invited to accept astronomical correction from the papacy mid-Reformation, largely preferred to be wrong with Caesar than right with the pope (the astronomer Kepler’s quip to that effect became proverbial); Orthodox Europe agreed. For centuries the continent ran on split time: a letter could arrive before it was sent, ten (later eleven, then twelve) days separated London from Paris, and careful correspondents dated letters both ways. William Shakespeare and Miguel de Cervantes famously both died on 23 April 1616 — ten days apart.
Britain and its colonies converted only in 1752, deleting eleven days (2-14 September) and moving New Year from 25 March to 1 January — which is why George Washington’s birthday shifted, why the UK tax year still starts on the odd date of 6 April, and why folklore insists London mobs rioted chanting “Give us our eleven days!” The riots are essentially myth — grown from election satire, above all Hogarth’s painting of a stolen-days placard; documented grievances were about rents and wage arithmetic, not stolen lifespan. Russia held to Julian until the revolution — its “October Revolution” of 1917 falls in Gregorian November — converting in 1918; Greece, last in Europe, in 1923; the Orthodox churches’ liturgical calendars split over it and remain split, which is why many celebrate Christmas on Gregorian 7 January.
The reform’s precision has held: the Gregorian year errs by about one day in three millennia. The deleted days, meanwhile, remain history’s tidiest demonstration that time’s bookkeeping is a human artefact — maintained, like all infrastructure, by someone, and political the moment anyone touches it.