In October 1582, millions of Europeans went to sleep on Thursday, October 4th and woke up on Friday, October 15th! The 10 missing days corrected centuries of accumulated solar drift in the Julian calendar.
The Historical Leap Error of Julius Caesar
Julius Caesar's calendar in 45 BC assumed the solar year was exactly 365.25 days. Adding 1 leap day every 4 years was off by 11 minutes per year, leading to a 1-day drift every 128 years. By 1582, the spring equinox had drifted from March 21 to March 11.
Modern age arithmetic relies on the perfected Gregorian system. Verify your age on AgeCalculate.net.