This is for those of you who absolutely hate to celebrate birthdays. The problem is you would have to have been born in Sweden in 1712, on a day which was officially listed in the calendar as February 30, 1712. Since there has never been another February 30 in Sweden, you would have died before reaching your first birthday. No cake or presents or Happy Birthday songs for you! (I wonder how many Swedes were so affected.)
According to Futility Closet, where I saw this story, Sweden had some calculation problems in switching from the Julian to the Gregorian calendar. At first, they omitted all leap days between 1700 and 1740, but in 1712, hey decided to have both a February 29 and a February 30. It was not until 1753 that the Gregorian calendar was fully implemented. Until then, there remained a lot of confusion.
Speaking of which, has Sweden ever had any notable mathematicians. Just wondering.