The Gregorian Calendar became effective on September 14, 1752. The previous day was September 2, 1752.
This change added 12 days to previous dates. It corrected the error in the calendar. In addition, the previous leap year
rule was modified (that rule made any year that is divisible by 4 a leap year, except those years divisible by 100).
Under the new calendar, the previous leap year rule would continue except that years divisible by 400 would now be leap years. In addition, the first day of the year was changed from March 25th to January 1st.
Therefore, anyone born before March 25th had 1 year added to their birth date, and everyone had 12 days added.