Scientists are delaying the start of 2006 by the first "leap second" in seven years, a timing tweak meant to make up for changes in the Earth's rotation.
The adjustment will be carried out by sticking an extra second into atomic clocks worldwide at the stroke of midnight Coordinated Universal Time, the widely adopted international standard, the US National Institute of Standards and Technology said this week.
"Enjoy New Year's Eve a second longer," the institute said in an explanatory notice. "You can toot your horn an extra second this year."
Coordinated Universal Time coincides with winter time in London. On the US East Coast, the extra second occurs just before 7pm (10am AEDT) on New Year's Eve. Atomic clocks at that moment will read 23:59:60 before rolling over to all zeros.
A leap second is added to keep uniform timekeeping within 0.9 second of the Earth's rotational time, which can speed up or slow down because of many factors, including ocean tides. The first leap second was added on June 30, 1972, according to NIST, an arm of the US Commerce Department.
Since 1999 until recently, the two time standards have been in close enough synch to escape any need to add a leap second, NIST said.
Although it is possible to have a negative leap second – that is, a second deducted from Coordinated Universal Time – so far all have been add-ons, reflecting the Earth's general slowing trend due to tidal breaking.
Deciding when to introduce a leap second is the responsibility of the International Earth Rotation and Reference Systems Service, a standards-setting body. Under an international pact, the preference for leap seconds is December 31 or June 30.
Precise time measurements are needed for high-speed communications systems