Take a Leap Today (Feb 29, 2008)

If it weren't for the extra day every four years, then the calendar would fall short by one day as the solar year is about 365.242190419 days depending on the source. The decimal day translates to just under six hours, or 11 minutes short of 365 and a quarter days.

Our current Gregorian calendar has an average year of 365 + 97/400 = 365.2425 days.

Since the Gregorian calendar is a bit faster than the solar year, the rule of adding leap days every four years had to be modified from the old Julian calendar so that extra leap days would not appear in years that are divisible by 100 unless they are divisible by 400, meaning, the years 100, 200, 300, 500, 600, 700, 900, 1000, 1100, 1300, 1400, 1500, 1700, 1800, 1900, and 2100 are not leap years. The years 400, 800, 1200, 1600, and the one we just experienced eight years ago, 2000, are leap years.

The longest time between two leap years is eight years. The last time that occurred was between 1896 and 1904. The next time will be between 2096 and 2104. This meant that people born on Leap Year day in 1896 didn't celebrate their actual birthday until eight solar years later. The few that may still living would be 112 years old.

The Julian calendar first used by the Romans and still in use by some countries is fast by 13 days from the day after their leap year day in 1900 until their leap year day in 2100. They add an extra day to February in every year divisible by four regardless. This rule gives an average year length of 365.25 days. However, it is 11 minutes longer than a real year. This means that the vernal equinox moves a day earlier in the calendar every 131 years.

From the day after Julian leap year day 1800 to leap year day 1900, the difference between the Julian and Gregorian calendars were 12 days. From 1700 to 1800, it was 11. From 1500 to 1700, it was 10. When some countries switched from the Julian to the Gregorian calendars beginning in 1582, they had to drop the number of days if they were in a certain year range to get the correct date for the Gregorian calendar. Italy, Poland, Portugal, and Spain adopted the Gregorian calendar in 1582. In Great Britian and America, it was adopted in September 1752.

The Gregorian calendar was designed to keep the vernal equinox on or close to March 21, so that the date of Easter (celebrated on the Sunday after the 14th day of the Moon that falls on or after 21 March) remains correct with respect to the vernal equinox.

On This Day in Sweden (Feb 30, 1712)

And here's the reason for that.

In the year of 1700, Sweden, which included Finland at the time, planned to make a slow conversion from the Julian to the Gregorian calendar. Sweden decided to create its own Julian-to-Gregorian calendar or whatever it was called by dropping the leap year day in 1700, which was a leap year in the Julian calendar. By error, the years of 1704 and 1708 were leap years, so that Sweden was one day fast of the Julian calendar. This confused travelers, wondering what day it was when they visited Sweden.

Sweden's modified Julian calendar was a day ahead of the actual Julian calendar, but ten days behind the Gregorian calendar.

So in 1712, Sweden added a bonus leap year day at the end of February, February 30th, to bring their calendar back in line with the Julian calendar. The February 30th in Sweden was February 29th in the Julian calendar and March 11 in the Gregorian calendar in 1712. Therefore, the skipped Leap Year Day in 1700 was applied in 1712.

Sweden’s final conversion to the Gregorian calendar occurred in 1753, when an 11-day correction was applied so that February 19 became March 1 that year. So the day after February 18th was March 1st.

Going to modern times, there is a 13 day difference between the two calendars, so February 27 Julian corresponds to March 11 Gregorian, Feb 28 Julian to Mar 12 Gregorian, Feb 29 Julian to Mar 13 Gregorian, and so forth until the Julian calendar hits their Leap Year Day in 2100, in which Feb 29, 2100 Julian corresponds to Mar 14 in Gregorian.

Soviet Had a Feb 30 For Two Years in 1930 and 1931

Source: http://www.timeanddate.com/date/february-30.html

Wikipedia reports that the 13th century scholar Sacrobosco claimed that in the early Julian calendar, February had 30 days in leap years between 45 BC and 8 BC. Augustus later shortened February to give the month of August named after him the same length as the month of July named after his adoptive uncle Julius Caesar, therefore, July and August has 31 days. However, all other historical evidence relating to the Julian calendar during this period refutes Sacrobosco, including dual dates with the Alexandrian calendar.

Was There Ever a February 31st?