The world's official timekeepers added a "leap second" to 2008, to match clocks to the Earth's slowing spin on its axis, which takes place at ever-changing rates affected by tides and other factors
The extra second was added at 23 hours, 59 minutes 59 seconds GMT on 31 December - just before 1pm on Thursday New Zealand time.
The "leap second" has been used sporadically at the Royal Observatory at Greenwich in Britain since 1972 to account for the minute slowing of the earth's rotation - tweaks that have kept Greenwich Mean Time the internationally agreed time standard.
Since the exact speed of the earth's rotation can't be plotted out in advance, the extra seconds are added as needed.
Sometimes they are added on 31 December, at others, they are inserted on 30 June.
Those willy-nilly fixes can trip up time-sensitive software, particularly in Asia, where the extra second is added in the middle of the day.
Critics say everything from satellite navigation to power transmission and cellular communication is vulnerable to problems stemming from programmes ignoring the extra second or adding it at different times.
Some scientists say GMT should be replaced by International Atomic Time - computed at the International Bureau of Weights and Measures near Paris - because new technologies have allowed atomic time to tick away with down-to-the-nanosecond accuracy.
But opponents say atomic time's very precision poses a problem.
A strict measurement, they say, would change our very notion of time forever, as atomic clocks would one day outpace the familiar cycle of sunrise and sunset.
The time warp wouldn't be noticeable for generations, but within a millennium, noon - the hour associated with the sun's highest point in the sky - would occur around 1 o'clock. In tens of thousands of years, the sun would be days behind the human calendar.
When the extra second is added, the National Institute of Standards and Technology in Boulder, Colorado, which provides the time standard for computers and appliances across the United States, will probably be busy fixing bugs starting at 5pm Mountain Time, predicted Judah Levine, a physicist at the institute's Time and Frequency Division.
"There's always somebody who doesn't get it right," Levine said. "It never fails."