The Missing October Days of 1582 | Generated by AI
Question: Why in October 1582 does the iOS calendar show October 4 followed directly by October 15, missing 10 days?
Answer:
The iOS Calendar (and many other digital calendars) accurately reflects a real historical event: the transition from the Julian calendar to the Gregorian calendar in 1582.
The Julian calendar, introduced by Julius Caesar in 45 BCE, slightly overestimated the length of a solar year (by about 11 minutes). Over centuries, this caused the calendar to drift out of alignment with the seasons—most importantly, shifting the spring equinox (key for calculating Easter) earlier than intended.
By the 16th century, the drift had accumulated to about 10 days. To correct this, Pope Gregory XIII introduced the Gregorian calendar (the one we still use today). The reform:
- Skipped 10 days to realign the calendar with the solar year.
- Changed the leap year rules (century years are leap years only if divisible by 400) to prevent future drift.
The change was implemented in Catholic countries starting in 1582: Thursday, October 4, 1582, was immediately followed by Friday, October 15, 1582. The dates October 5 through 14 simply did not exist in those regions under the new system.
October was chosen for the skip because it had fewer major religious holidays, minimizing disruption.
Apple’s Calendar app models this historical transition precisely, so when you scroll back to October 1582, it jumps from the 4th to the 15th—just as it actually happened.
References:
- https://www.britannica.com/story/ten-days-that-vanished-the-switch-to-the-gregorian-calendar
- https://en.wikipedia.org/wiki/Gregorian_calendar
- https://www.iflscience.com/why-october-is-missing-10-days-in-the-year-1582-on-your-phone-74872
- https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/DatesAndTimes/Articles/dtHist.html (Apple’s documentation on historical calendar modeling)