This year will be longer than usual -- by one second, the U.S. Institute of Standards and Technology said Wednesday.

The earth is sufficiently out of sync that a leap second has been scheduled for 7 p.m. U.S. Eastern Standard Time on Dec. 31, said the institute, noting those interested in watching it happen should go to www.time.gov before midnight, London time, and click on their time zone.

A total of 24 leap seconds have been added since 1972, the last being in December 2005, because the earth is slowing and does not rotate exactly once every 24 hours, or 86,400 seconds, the Institute said.

The discrepancy went unnoticed until highly accurate atomic clocks were developed in the late 1960s. It was decided then, by international agreement, that operators of atomic clocks around the world would adjust the time of day by adding one second to the world's official time when needed.