George Washington's two birthdays

Pope Gregory XIII (Wikimedia Commons)

In the Washington family bible, it lists George Washington's birth date as "the 11th day of February 1731/2." This strangely written year is because of the change from the Julian calendar to the Gregorian calendar.

The Julian calendar was named after Julius Caesar who developed a calendar that was 364 1/4 days and closely matched the time it takes the earth to go around the sun from the beginning of spring to the beginning of spring (which was March 25th and when the year changed). Caesar's calendar was 365 days long, but included an extra day, or leap day, every four years to account for the extra 1/4 day.

Though this solved the problems in older calendars that were used, it wasn't quite accurate enough because the actual time between the beginning of one spring to the beginning of the next spring is 365 days 5 hours 48 minutes and 46 seconds. This means the Julian calendar was 11 minutes and 14 seconds too slow. This added up to a full day off every 128 years.

By the time Pope Gregory XIII came around in 1572, the calendar was off by ten days, so the first day of spring was happening earlier and earlier on the calendar but really should have been happening on the vernal equinox (Latin for "equal night"), which is the date in the spring when there is the same amount of daylight and darkness.

Pope Gregory did two things to fix the problem. First, though he kept the 365 day calendar with a leap day every four years, he added a new rule: there would be no leap year in years ending in "00" unless those years were divisible by 400. Therefore, the years 1700, 1800, 1900, and 2100 would not be a leap year but the years 1600 and 2000 would. This change was so accurate that today, scientists need only add leap seconds every few years to the clock in order to keep the calendar correct. Secondly, since the Julian calendar had fallen ten days behind over the centuries, Pope Gregory XIII said that October 4, 1582 would be officially followed by October 15, 1582. Not only would the new calendar be used, but ten days would be "lost" forever, and the new year would now begin on January 1 instead of March 25.

What does this have to do with Washington's birthday?

Though Pope Gregory's new "Gregorian calendar" was developed in 1572, it took centuries for it to be adopted worldwide. Great Britain finally decided to adopt the calendar in 1751 and their government said September 2, 1752 would be followed by September 14, 1752. Because they took so long to change to the new calendar, they had to add eleven days instead of ten. At that time, America was under British rule, so the American colonies changed their calendar then, too.

Because people were so aware of the changes, for many years after, dates were written using both the old and the new calendars. Before 1752, the new year began March 25, and after, it changed to January 1st. Therefore, Washington's birthday was in 1731 on the old calendar and 1732 on the new calendar since it was in February. And, since his birthday on the old calendar was February 11 and they added eleven days to the calendar, his birthday became February 22nd.