WASHINGTON — With a brutal economic slowdown, 2008 may feel as if it will never end. Now the world’s timekeepers are making it even longer by adding a leap second to the last day of the year.

Along with the economy, the Earth itself is slowing down, requiring timekeepers to add an extra second to their atomic clocks to keep in sync with Earth’s slightly slowing rotation. So an extra second will be tacked on to Dec. 31 after 6:59:59 p.m. and before 7 p.m. Eastern Standard Time.

That extra second will make 2008 — already long with an extra day on Feb. 29 — the longest year since 1992.

The decision to add an extra second was made by an international consortium of timekeepers, whose American arm announced it Monday. World commerce and digital technology depend on accurate to-the-second timekeeping, said Geoff Chester, spokesman for the U.S. Naval Observatory in Washington, responsible for one-third of the world’s atomic clocks.

Most cellular phone providers and computer operating systems check with the world’s atomic clocks and update their time to add the leap second automatically, he said.

The world started adding leap seconds in 1972, sometimes twice a year. This is first leap second since Dec. 31, 2005. This is the fourth year to have a leap day and a leap second.

At the Naval Observatory they have a party at 6:59:60 p.m.

“We watch the clock and make sure nothing breaks,” Chester said. “It’s an early New Year’s celebration.” A brief one.

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The U.S. Naval Observatory