The adjustment will be carried out by sticking an extra second into atomic clocks worldwide at the stroke of midnight by Co-ordinated Universal Time, the widely adopted international standard, the US National Institute of Standards and Technology said.

 

"Enjoy New Year's Eve a second longer," the institute said in a notice. "You can toot your horn an extra second this year."

 

Co-ordinated Universal Time coincides with winter time in London. On America's East Coast, the extra second occurs just before 7pm 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 seconds 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 30 June 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.

 

Tidal braking

 

Although it is possible to have a negative leap second - that is, a second deducted from Co-ordinated Universal Time - so far all have been add-ons, reflecting the Earth's general slowing trend due to tidal braking.

 

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 31 December or 30 June.

 

Precise time measurements are needed for high-speed communications systems among other modern technologies.