In January 2012, the International Telecommunication Union (ITU) will hold a meeting to decide the fate of leap seconds. Leap seconds have long been a subject of debate, and some organisations have argued that our increasing dependence on precision timekeeping technology makes the leap second system costly and difficult to implement.

Technologies that produce or require precise timekeeping currently have to be adjusted whenever a leap second is added, a process that leaves the systems vulnerable to human error. But if leap seconds were removed, the length of a 'clock day' (86,400 SI seconds) would slowly drift away from the 'solar day' defined by the movement of the sun across the sky.

What is the difference between GMT and UTC, and where do leap seconds come in?

Greenwich Mean Time (GMT) is based on the Earth's rotation. A mean solar day is defined so that on average over a year, the Sun is directly overhead on the Greenwich meridian at noon, and that day is then broken down into hours, minutes and seconds. However, the rotation speed of the Earth fluctuates unpredictably, and in the long term is slowing down due to friction caused by the ocean tides. This means that the length of the mean solar day is not consistent enough for use in precise timekeeping.

Time scales based on atomic clocks provide a much more precise method of timekeeping than the Earth's rotation, but at the moment the Earth's rotation is a little slower than atomic time, and a 'GMT year' is around half a second longer than a 'clock year'. The time scale that forms the basis of all precise timekeeping worldwide is called Coordinated Universal Time or UTC, and is a compromise between solar time (GMT) and atomic clock time. In order for the time kept by atomic clocks to keep in step with GMT, leap seconds are added to UTC at a rate of more or less one every couple of years. Leap seconds can be either positive or negative, as the unpredictable changes in the Earth's rotation can be greater than the overall slowdown.

What would happen to the time system if leap seconds were removed?

All time signals are already linked to UTC and all countries base their civil time on UTC. Losing leap seconds would mean that clock time and solar time would no longer be linked, and would slowly drift apart by around half a second each year. UTC would no longer provide an approximation to GMT, and the UK's laws would have to refer to UTC rather than GMT. The link between the Earth's rotation and the time of day would be lost, and the most likely outcome is that countries would eventually (after several hundred years) decide to change their time zone to correct the difference that will build up between the solar day and clock time.

What are the arguments for removing leap seconds?

The leap second procedure was introduced in 1972, at a time when no one could have predicted our current need for high-precision timekeeping. The move away from celestial navigation has greatly reduced the need for civil timekeeping to be closely linked to the Earth's rotation. With civil time already offset by an hour or more from the local mean solar time in many places, due to time zones and, in some countries, 1-hour daylight saving time changes twice a year, it is argued that most people will not notice a small, very slow drift of civil time away from solar time if leap seconds are abandoned.

Complex timekeeping systems and software have to have leap seconds added to them manually. As the Earth's rotation is irregular, leap seconds do not follow a simple pattern (unlike leap years) so each leap second has to be programmed in separately after it is announced, with only six months' notice. This can be a difficult and costly process, and it means that human error could affect timekeeping systems (although there have not been any reports of this as yet).

What are the arguments for keeping leap seconds?

Losing leap seconds would mean losing the link between the Sun and time, however small the drift is – and eventually, although it might take tens or hundreds of years, the drift would be substantial enough to be noticeable. Astronomers and earth scientists making measurements based on the Earth's rotation would have to compensate for the loss of leap seconds in their systems, which would also be a manual and difficult process. There is also concern that ending leap seconds would have the effect of redefining the day to be 86,400 UTC seconds, and the gradual divergence between the UTC day and the mean solar day might have many social and legal consequences that have not yet been identified.

Some countries, including the UK, have argued that the leap seconds procedure has been in use for 40 years and continues to work adequately well, and that the difficulties reportedly caused by leap seconds should be dealt with by technical solutions rather than by changing the fundamental basis of civil timekeeping. Systems such as GPS are already in place to provide precise, atomic clock-based timekeeping standards without the implementation of leap seconds – those whose work needs a consistent time scale can use these systems without disrupting the current international time system.

Are there any other options?

It has been suggested that leap seconds could be replaced with larger steps in UTC occurring less frequently (for example, inserting a leap minute every few decades). However, a 2004 proposal from the USA to replace leap seconds with leap hours was rejected for a number of reasons, including concerns that a leap hour would create far more severe problems than a leap second. A change to any steps in UTC other than one-second steps is likely to be technically too difficult, too risky and too costly. Unlike summer time systems, where the underlying time scale (UTC) remains the same but an hour is added or taken away, a leap hour (or leap minute) would change this reference time scale entirely. Any software or clocks (including atomic clocks) dependent on the time scale would have to be upgraded to deal with this change, and coordinating a noticeable worldwide time shift would be challenging for developers and disruptive to users.

If leap seconds are ended, we could keep civil time aligned with mean solar time by changing our time zone by one hour whenever the difference between UTC and GMT becomes sufficiently large (it would take roughly 600 years for the offset to reach 30 minutes, when the first time zone change would be needed). This would simply involve not putting the clocks forward for summer time in the chosen year.

Where does the UK stand?

The UK opposes the proposal to end leap seconds in UTC. It has reviewed the evidence presented to the ITU of difficulties caused by leap seconds, and concluded that the reported problems are relatively minor and in many cases could be reduced or eliminated by improved procedures for handling leap seconds. Consultations with Government agencies and professional bodies that have an interest in precise timekeeping found no support for the proposal but strong opposition from some quarters.

What will happen next?

The proposal to end leap seconds in UTC will be discussed and voted on at a meeting of the ITU Radio Assembly in January 2012. If the proposal is accepted, there will be no further leap seconds in UTC after an agreed date (possibly 2017) and UTC will then gradually drift away from GMT. If the proposal is rejected, leap seconds will continue, although there are likely to be changes to reduce the difficulties that they cause. For example, the maximum allowed offset between UTC and GMT could be increased so that leap seconds can be announced several years in advance rather than six months.