For 434 years, the same instant had different dates depending on where you were standing. Pick a day and see what each calendar called it.
Ten dates that never happened. The 5th through the 14th were not skipped over — they were never assigned to any day. Nothing occurred on them, anywhere, because they did not exist. Your computer disagrees: most date libraries use the proleptic Gregorian calendar, which projects today's rules backwards forever and will happily hand you 10 October 1582.
The Julian/Gregorian gap moves the day. This moves the year, and it is a completely separate problem stacked on top. England began its legal year on 25 March — Lady Day — until the same 1752 act that skipped the eleven days also moved the new year to 1 January. So a parish register reading 20 February 1719 means 1720 to us.
Two consequences worth seeing. First, contemporaries knew it was confusing and wrote both: Pepys dated a diary entry 1 January 1661/62. Second — and this is the one that breaks people reading old documents — “24 March 1700” falls nearly a year after “25 March 1700.” Same year label, 364 days apart, because the label only changes at Lady Day. Try both in the box above with the year starting 25 March.
Washington is the clean example, because both numbers move for different reasons. He was born 11 February 1731 as written. The day shifts eleven places for the calendar gap; the year shifts because February fell before the old new year. We call it 22 February 1732.
And it is not over. Scotland moved its new year to 1 January in 1600 and England did not follow until 1752 — so for the 149 years after the crowns united, one monarch's two kingdoms disagreed about what year it was for roughly three months of every year. The 1752 act shifted the quarter days forward by eleven days so nobody gained or lost a rent period, which is why the UK personal tax year still ends on 5 April: Lady Day, 25 March, plus the eleven deleted days. A 274-year-old correction, still load-bearing in a live government deadline.
The Julian year is 365.25 days. The mean tropical year — one full cycle of seasons — is about 365.2422. The calendar therefore ran fast by roughly 11¼ minutes a year, which is one whole day every 128 years. The size of the correction tells you what they were correcting to: ten days points back to AD 325, the Council of Nicaea, where the rule fixing Easter to the equinox was set. Calibrating to the start of the era instead would have required twelve.
Between the first adoption and the last, 434 years. During that span a letter could be dated by its sender and arrive under a different date entirely — which is why historians write dates "O.S." and "N.S.", old style and new style, and why a date without a place is an incomplete claim.