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Historic dates depend on location
Skipped Dates
When one country moved from the Julian calendar to the Gregorian calendar in the 18th century, it did not simply lose about three weeks. It skipped right from one date to another and missed the ones in between. A program that works out how many days apart two historical dates are has to know exactly which dates vanished.
Russia's Later Switch
Russia only changed to the Gregorian calendar in the 20th century, so its skipped dates are different. The dates that disappear depend on the location.
Britain's March Year
Britain adds another wrinkle: until, I think, the 16th century, the year started on the 25th of March. That puts the day after the 24th of March at the 25th of March of a new year, so the numbers on the calendar do not run in the order a modern reader expects.
Why did countries adopt the Gregorian calendar at different times?
Pope Gregory XIII introduced the calendar in 1582, and Catholic countries such as Spain, Portugal, and Italy adopted it at once. Protestant and Orthodox regions often waited. Britain and its colonies switched in 1752, dropping 11 days, so September 2 was followed by September 14. Russia switched after its 1917 revolution, in 1918, dropping 13 days.
Why did the Julian calendar need fixing?
The Julian year is about 11 minutes longer than the solar year. Those minutes add up to roughly one day every 128 years. By 1582 the spring equinox had drifted about ten days from where the Church's calendar placed it. The Gregorian reform dropped ten days and changed the leap year rule so century years are leap years only when divisible by 400.
When did the English year begin on March 25?
In England the civil and legal year began on Lady Day, March 25, which marks the feast of the Annunciation. Dates between January 1 and March 24 were often written with two years, such as 1731/32, to show both usages. The practice ended when Britain adopted the Gregorian calendar and made January 1 the start of the year. Historians still see the double years in old parish registers.
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