From iflscience:
People on social media have noticed an odd quirk in the iPhone calendar: if you scroll back to October 1582, you’ll see it jump straight from the 4th to the 15th, skipping ten entire days. It’s not a glitch or a hidden joke from Apple’s developers; those ten days genuinely never existed.
The days weren’t erased from the passage of time using a cosmic cut-and-paste tool. Instead, people in 1582 went to bed on the 4th and woke up on the 15th (not that much of the world would have realized at the time). What Happened in October 1582? As IFLScience explained in 2023, the Catholic church adopted the Gregorian calendar in October 1582. Prior to this, most of Europe had used the Julian calendar, introduced by Julius Caesar in 45 BCE. To understand why, we must go back to the 16th century when a major shift occurred in the way we organize days, weeks, months, and years.