HK gets set to add a leap second

PUBLISHED : Sunday, 13 May, 2012, 12:00am
UPDATED : Sunday, 13 May, 2012, 12:00am


The flag to mark the 15th anniversary of Hong Kong's handover to the mainland will be raised a bit late - by one second.

That is because at 7:59:59 am on July 1, Hong Kong time, a leap second will be added, in accordance with the addition to co-ordinated universal time (UTC), the primary standard by which the world regulates clocks.

The police officer responsible for raising the flag - which is supposed to be at the top of the pole at 8am - will have to be sure his watch is in synchronisation with UTC, and not the mean solar time, which is based on the earth's rotation.

So the first day chief executive-elect Leung Chun-ying takes office will be a second longer.

A leap second is an extra second added every few years to keep time measured by atomic clocks in sync with the time based on the earth's rotation, which has been slowing.

The Hong Kong Observatory took care to announce the news last month - more than two months in advance this time because some companies had problems with their computer servers on previous occasions, according to the Observatory's scientific officer, Woo Wang-chun. It used to do so two weeks prior to when the extra seconds were added or subtracted.

The caesium beam atomic clock in the Observatory's headquarters in Tsim Sha Tsui, which keeps Hong Kong time accurate, will be programmed to display 7:59:59, 7:59:60, then 8:00:00, Woo said.

Since 'clocks aren't usually programmed to show 60 seconds', he and his colleagues have been running tests to ensure the computer systems in the Observatory will not break down.

The city's 'most accurate' clock maintains an accuracy better than a millionth of a second a day, and is checked by many services in the city, including government bureaus, so the change will have a ripple effect.

A leap second could disrupt a vast range of modern navigational and communications systems, such as satellite navigation, financial services, air traffic control and information technology companies.

'One second may seem a short time,' Woo said. 'But it can cause a lot of problems, because this second is unpredicted - it came out of nowhere. We have heard from companies that some of their programs stopped working because of that added second. It is a big deal.

'How can we describe that one second to a computer?'

Computers traditionally accommodate leap seconds by setting their clock backwards by one second, but this repeated second can be a problem. For example, what happens to operations that take place during that second? Does e-mail that comes in during that second get stored correctly? How does the database record what happens during that second?

'Many databases need to have a continuous record of what happens to help them make decisions,' Woo said. 'If we don't have a clear definition of the second, we may risk losing the complete account.'

Woo called on companies in the city to see to the potential problems it could create, though he said ordinary people would not have to do anything and would not even notice it.

Despite the early announcement, it would not surprise Woo if some companies had hiccups because, to his surprise, some of them had indeed reported incidents even during the once-in-four-years leap year.

Technology giant Google, in dealing with the issue, has modified servers to 'gradually add a couple of milliseconds to every update, varying over a time window before the moment when the leap second actually happens'.

Its clustered systems stopped accepting work on a small scale during the leap second in 2005, though no serious damage was done, it said on its blog in September.

In another instance, in 2003, a leap-second bug made global positioning system receivers from Motorola briefly show customers the time as half past 62 o'clock.

Countries such as the United States, France and Germany want to abolish the leap second, but Britain, China and Canada want it to stay.

That is a matter for time experts at the International Telecommunication Union to decide in 2015, as they did not reach a consensus in January. Those who want to lose the leap second say that the one-second additions are becoming increasingly problematic and require a lot of work.

The International Bureau of Weights and Measures in Paris has said that losing the leap second could cause long-term problems.

And most Chinese scholars think it important to maintain a link between civil and astronomical time due to Chinese tradition.

'Much of the disagreement between countries concerns their mindsets and cultures,' Woo said. 'For instance, [Britain], the birthplace of Greenwich Mean Time, thinks what has been done for thousands of years should be continued.'

Hong Kong does not have a say on this because the city is not a sovereign state, but Woo admitted that keeping a link between the earth's rotation and the atomic clock meant extra work for many people, especially those working in the information technology sector.

The leap second was introduced in 1972 to keep our modern timekeepers - atomic clocks, which rely on the vibrations in atoms to provide a very accurate measurement of time - in line with the natural yet slightly less reliable timekeeper, the earth. Because the planet wobbles a little on its axis as it spins, it means some days end up being a few milliseconds longer or shorter than others.

Other factors, including earthquakes and volcanic eruptions and being dragged around by gravitational forces, make the planet's rotation erratic too, but in general it has been slowing down.

This means that over time, atomic clocks and the time based on the earth's rotation will drift further apart.

Over decades, this would amount to a minute's difference, but over 500 years it could be an hour, and over thousands of years the sun could be setting when atomic clocks claim it is morning.

When this difference is deemed by the International Earth Rotation Service, which monitors the earth's activity, to be approximately 0.9 seconds, a leap second is added to pull the two back into synchronisation.

Sometimes it can be added on consecutive years, sometimes not for several, with six-months' notice given before action needs to be taken. There have been 24 leap seconds added in history.

The rift is not the first time that leap seconds have been brought to the time community's attention: in 2005, the US proposed that the leap second should be abolished, and replaced with a leap hour, but this failed to be passed by the International Telecommunication Union members.


Years, nearly, that a study panel has been working for the ITU on the issue of whether to eliminate leap seconds, without consensus