February 30, 1712 was a completely valid date... in Sweden at least. They were out of phase with the Julian calendar by a day, so they made an extra leap day in February.

It would have sucked to have this day as your birthday.

February 30 was a standard part of the Julian calendar as originally devised, and occurred in a few of the years of the early Roman Empire, and would still be with us were it not for the vanity of the Emperor Augustus.

Julius Caesar, advised by the astronomer Sosigenes, instituted a reformed calendar, using the same old twelve month names, but starting on January 1 instead of March 1, and with a change to the numbers of days in each month: they alternately had 31 and 30, so January had 31, February had 30, March had 31, and so on.

This produces a year of 366 days, which was a lot closer to the true year than their previous reckoning, but still too high by three-quarters of a day, so Julius and Sosigenes declared that February 30 was a leap day, only occurring every fourth year, and that in other years February would have only 29 days. This was the working, regular Julian calendar we should have had until the Gregorian reform of the 1500s.

Enter first some assassins, then some incompetent priests, and finally a godlike emperor. The new calendar began in the Roman year 709 AUC, or what we now call 45 BCE or BC. Caesar was assassinated in the following year. The priests in charge of keeping it up then read "every fourth year" in the Roman way, including the years at both ends: e.g. 2000, 2001, 2002, 2003 is four years, so if 2000 is a leap year then so is 2003.

Incidentally this Roman counting system is why a quartan fever is one that recurs every three days (as we would put it); and why Jesus, crucified on a Friday, was said to have been resurrected on the third day, a Sunday: in English we'd more naturally say "two days later".

So February 30 occurred every third year for a while. (I can't tell you which years exactly were leap years, because I don't know whether 45 BCE was; nor do I know when the correction I'm about to mention happened exactly.)

The old fifth month, Quintilis, now the seventh month, was renamed Julius after the late dictator. His successor Augustus was deemed worthy of the same honour, and the now eighth month Sextilis was renamed with his name. The problem was that while July had 31 days, August had only 30, which was not fitting for the majesty of the emperor, so they transferred a day from February, and mucked around with the lengths of the later months. So sometime during the reign of Augustus, February 30 was lost altogether, and February 29 became the once every four years leap-day.

Around the same time they also discovered the error of having leap days every third year, and fixed this by omitting several scheduled leap years. The Julian calendar finally got into its correct form in the modern shape we use in the year we call 8 CE (or AD). Although several later emperors (stand up, Nero) renamed months for themselves, none of them did any more tampering with the actual days. So we had to wait until 1712, and then only in Sweden, for another February 30.

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