The Y5K bug? Hurdling the leap day problem

Our modern "Gregorian" calendar has been used by the Roman Catholic Church since 1582 and was adopted by England in 1752. But University of Maryland physicist David Book points out there are still a couple of problems with the system. In a recent letter to the Washington Post, he mentions one he calls the "Y5K problem."

Our modern calendar is designed with the seasons in mind. It is supposed to match up with what a farmer considers a year, or the time between two vernal equinoxes. (The vernal equinox, which marks the start of spring, occurs on the day in which the Sun rises over Earth's equator.) Using this definition, a year is 365.2422 days. By adding leap years at regular intervals, our modern calendar sets an average year at 365.2425 days -- very close to the length between vernal equinoxes but with a slight discrepancy of .0003 days.

The discrepancy may be small, but it adds up. Geoff Chester, with the US Naval Observatory (whose Time Service Department keeps the official time for the United States) points out that we are already "three hours ahead of what the seasons tell us." If we do nothing, our calendar will be a day ahead of schedule in 2915 years. Book calls this the "Y5K problem," because it would occur roughly around the year 5000.

Book also says that we need to think about fixing the problem now. "These errors accumulate. The longer we wait to make the correction, the more disruptive it will be." There are even suggestions floating around for how to make the change.

Chester says he's seen "one or two proposals to refine the Gregorian calendar by one leap day." In one such proposal, he explains that we would continue with the "normal" Gregorian rules with one exception -- years divisible by 4000 (such as 4000 AD and 8000 AD), which are leap years under the present system, would become ordinary years. "Thus, you'd have 969 leap days every 4000 years," says Chester, "resulting in an average calendar year of 365.24225 days." With this method, Chester days "we won't get a full day out of kilter for some 15,000 years." Whether or not the Gregorian calendar will actually be changed, however, only time will tell.