Nº 7 2013 > The future of time
To abolish or not to abolish the leap second?
A British perspective of the future of Coordinated Universal Time
Peter Whibberley, Senior Research Scientist, Time and Frequency Group, National Physical Laboratory, United Kingdom
For more than 40 years, the current international reference time-scale, UTC, has been adjusted in occasional one-second steps known as leap seconds so that it remains closely aligned with time based on the Earth’s rotation. For the past 14 years, arguments have surged back and forth in international committees, particularly within the ITU Radiocommunication Sector (ITU–R) Working Party 7A, about the desirability of ending these leap-second adjustments. Despite lengthy debate, no consensus has been found, and an attempt to reach a decision at the World Radiocommunication Conference in 2012 (WRC‑12) led instead to a call for further studies.
The importance of UTC is that it forms the basis of civil timekeeping in the large majority of countries worldwide. It is a compromise, combining the stability obtained from averaging large numbers of atomic clocks in timing institutes around the world with a realization of mean solar time maintained by those leap-second adjustments. At the core of the debate is disagreement about the continuing need to maintain the close linkage between civil timekeeping and Earth time.
The UK Government has considered the arguments for and against ending leap seconds on several occasions. Every time, it has concluded that the present definition of UTC with leap seconds is satisfactory and that the known difficulties caused by leap seconds do not justify breaking the close link between civil timekeeping and the Earth’s rotation.
Debate in the United Kingdom about the future of UTC
The first formal proposal to end the insertion of leap seconds in UTC was submitted to ITU–R Working Party 7A in 2004.
To inform its decision-making, the responsible UK Government department consulted official bodies and agencies with an interest in precision timekeeping. None of these authorities reported significant problems arising from leap seconds, while some scientific institutions reported strong support among their memberships for retaining leap seconds. The results of this consultation process, together with the evidence collected by ITU–R Working Party 7A, were put to the Minister for Science at that time, who decided that the United Kingdom should oppose the proposed change. He considered that the reported problems caused by the application of leap seconds in UTC were insufficient to justify what was perceived to be a fundamental change to civil timekeeping.
The discussions within ITU–R on the future of leap seconds have continued to be monitored closely by the UK Government. On two further occasions, in 2008 and in 2011, the government minister responsible for scientific matters (a different person on each occasion) has been presented with an updated account of the debate and the evidence put forward by both sides. On both occasions, the minister was unconvinced by the arguments for ending leap seconds in UTC and decided to continue the policy of his predecessor.
It would be a mistake to assume that the UK Government’s dominant consideration is a desire to retain the name Greenwich Mean Time (GMT), which is often, inaccurately, applied to standard time in the United Kingdom rather than the correct term UTC. If leap seconds are ended and UTC is allowed to diverge from Earth rotation time, then the United Kingdom’s laws would have to be amended to refer explicitly to UTC rather than GMT. This change would require legislation, but would not be difficult to implement, and any adverse publicity arising from it would most likely be short-term. The lack of concern in the UK Government over the potential loss of GMT is illustrated by its support for proposals that would have moved the United Kingdom into the same time zone as the central European countries; one hour ahead of UTC in winter, and two hours ahead in summer. The resulting loss of the name GMT for United Kingdom’s civil time was not a significant factor in the debate.
There is particular concern in the United Kingdom over the lack of public awareness of the divorce between civil timekeeping and astronomical time that will occur if leap seconds are ended. A perception that time-of-day is intimately related to the Earth’s rotation is widespread, and the level of public opposition to ending that link is unknown. The proposed change might be viewed as an attempt by technocrats to impose an unnecessary and unpopular change, and studies of public attitudes to the proposal would be highly desirable.
Technical arguments for ending leap seconds
Although the detailed deliberations that underpin government decision-making are not made public, it is probably fair to say that the UK Government’s opposition to ending leap seconds arises in large part from its assessment that the evidence collected by ITU–R Working Party 7A of problems caused by leap seconds is insufficient to justify what is viewed as a fundamental change to civil timekeeping.
There is no doubt that when a leap-second adjustment is applied to UTC, costs are incurred in programming the leap second and some systems and equipment have difficulty in handling it. On each occasion when the UK Government has reviewed the available evidence, it concluded that the reported effects are relatively minor, and in many cases could be reduced or eliminated by improved procedures for handling leap seconds and increased automation to reduce the likelihood of human error. It was also concerned that ending leap seconds would itself create difficulties that have not yet been fully assessed.
A second argument put forward for ending leap seconds is that their presence in UTC has resulted in the undesirable proliferation of alternative unstepped time-scales, such as Global Positioning System (GPS) time. The UK Government is not convinced that this development is a cause for serious concern. These additional time-scales are only used internally within their respective systems, they are based on UTC, and they provide time outputs that are converted to UTC before being made available to general users.
If leap seconds are retained, they will tend to become more frequent over time. However, it would probably take well over a thousand years for the interval between leap seconds to decrease to one month, and the UK Government considers that there is plenty of time for further investigation of the long-term implications before a decision has to be reached.
A new name for Coordinated Universal Time without leap seconds
One of the most heated aspects of the debate concerns the retention of the name Coordinated Universal Time if leap-second adjustments are ended.
There are several arguments in favour of a new name. UTC without leap seconds would be a purely atomic time-scale, identical in nature to the existing International Atomic Time (TAI) and fundamentally different from a time-scale maintained within one second of astronomical time. For the past 80 years the term Universal Time has designated mean solar time, and a change of name would avoid confusion and ambiguity if the link is ended.
The argument that the name Coordinated Universal Time is now so deeply embedded in laws and regulations around the world that the effort needed to change the name could not be justified has little weight in the United Kingdom, where laws relating to civil time would have to be amended if leap seconds are stopped, regardless of the choice of name.
Alternatives to ending leap seconds
The development of the debate within ITU–R Working Party 7A has led to a polarization between only two options: eliminating leap seconds to change UTC into an unstepped time-scale; or retaining UTC in its present form. At various times other proposals have been put forward, and the current broadening of the debate may allow these alternatives to be considered more thoroughly.
One group of proposals involves retaining steps of some form in UTC, but modifying their size, frequency or scheduling to reduce their impact. Suggestions include leap hours and leap minutes, or accumulating leap seconds over a century. All of these ideas would achieve the aim of retaining a link between UTC and astronomical time. However, they all have the considerable disadvantage that all equipment and software in use worldwide that require precise time would have to be upgraded or replaced, and the cost and effort involved would be considerable. A larger, less frequent step might also cause greater disruption and have more severe consequences, and there is a widespread view in the United Kingdom that it would in practice prove impossible to implement any step larger than one second in UTC.
Other proposed solutions involve modifications to the present system. For example, an unstepped time-scale broadcast alongside UTC could be used in applications that cannot handle leap-second steps, and might be feasible if sufficiently robust methods can be developed to distinguish between the two time-scales. Improved protocols for handling leap seconds and clearer guidance for developers of timing systems would also be beneficial. The UK Government is keen to see such alternative approaches evaluated more thoroughly.
What’s the rush?
The existing scheme for leap-second adjustments of UTC has been in use for 40 years, with 25 leap seconds implemented, so a considerable body of experience in handling leap seconds has built up. Since the current procedures for inserting leap seconds could remain viable for many decades, there is no need to amend the present system in haste. Any misjudged change implemented now would be very difficult to reverse.
The UK Government has conducted high-level reviews of the arguments for ending leap seconds on three occasions, and each time has concluded that there is no compelling need to change the present system. UTC with leap seconds remains closely linked to the Earth’s rotation, which in the United Kingdom is widely considered to be an important requirement of the civil time-scale. Further studies and broader debate are essential for achieving the necessary level of support worldwide before any change is made to UTC.
In this article, Mr Whibberley summarizes the reasons why the United Kingdom has consistently maintained the position that the present definition of Coordinated Universal Time (UTC) with leap seconds is satisfactory, and that the known difficulties caused by leap seconds do not justify breaking the close link between civil timekeeping and the Earth’s rotation. All the comments and statements made in the article are Mr Whibberley’s interpretation of the debate, and not necessarily either his own personal opinions or those of the UK Government.