An extra day means an extra day of interest on your debts.
Today, February 29 - Leap Day -- is costing you an extra day's interest if you're repaying a debt. It's costing the U.S. an additional $3.5 billion in added national debt.* And if you're in jail, or waiting for your first of the month paycheck, it's adding an extra day to your wait.
It's also earning you a tiny bit more on your bank deposits and investments.
Whom do we have to thank -- or curse -- for this extra day every four years? Julius Caesar and his lover, Cleopatra.
In 48 BC, Julius Caesar was in Alexandria, Egypt, absorbing the culture and science -- and decadence -- of Cleopatra's capital. There he learned from an old sage named Acoreus about Egypt's calendar, which had a leap year.
At the time, the Roman calendar did not. Like most ancient calendars, it was based on the phases of the moon, which in one cycle takes about 29.5 days. But 12 months of 29.5 days doesn't equal the true length of the year as measured by the orbit of the Earth around the sun. It's off by 11 days, so anniversaries, holidays, and entire seasons drift backward on lunar calendars.
The ancient Egyptians had realized this and created a calendar 365.25 days long -- with the fraction averaged in by adding an extra day every four years.
When Caesar returned to Rome, he created a 365-day calendar with a quadrennial leap year, adding the extra day in February.
A minor hassle for some, perhaps, but certainly better than the alternative faced by the Romans. Back in 45 BC, for instance, their lunar calendar had drifted backward by 80 days --nearly three months. Spring had become winter, and autumn came in the summer months.
To correct this, Caesar decreed that 45 BC would be 445 days long. Think about the extra interest on 80 extra days! No wonder they called it the "Year of Confusion."
This post originally appeared in slightly different form exactly four years ago, on February 29 2008, on Portfolio.com. The information comes from my book Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year.
*This figure is updated for 2012.