This is a personal response to the UTC Questionnaire, as an individual contributor to the public domain UNIX timezone database at elsie.nci.nih.gov. I had originally not intended to respond to this questionnaire, since my experience is in the study of historical timezone changes and summer-time rules, and my views on the technical issues considered on their own are generally in accordance with those expressed in the response by Markus Kuhn, but on further consideration I decided that historical timezone research shows up potential practical, civil and legal issues that could arise with the decoupling of UTC from UT1, and of which it might be desirable to present a summary. In general, the time used for everyday purposes in most parts of the developed world (where time signals are readily available) is based on UTC, as distributed by time signals, with an offset that is normally an integral number of hours depending on the local timezone and summer-time arrangements. However, the legal definition of the time to be used may vary from country to country; for example, in the United Kingdom the time has been based on Greenwich Mean Time since 1880 (except that the whole of Ireland used Dublin Mean Time until 1916, then a single period of summer time at DMT plus one hour, then time based on Greenwich); while it has been proposed to change to a UTC-based definition, this sort of matter has not been a legislative priority. Although GMT has been replaced as a designation of a timescale by more precise definitions of variants of UT, it seems most likely that GMT would be taken, if UTC were decoupled from UT1, to be UT1 or very close to it, rather than UTC. In France, the time is now based on UTC, but from 1911 until 1978 the definition was based on Paris Mean Time, retarded by 9 minutes 21 seconds. The European Union directives defining the start and end points of summer time in the EU still state the relevant points in terms of Greenwich Mean Time. The time used in Liberia did not change to a Greenwich-based zone until 1972.

From this, it seems likely that many countries have definitions of legal time based on GMT rather than UTC; and, while the difference is insignificant for the purposes of everyday life, changing it has been of no great priority; a second here or there is not generally of great significance in the timing of everyday events. However, if UTC and UT1 are decoupled, it would seem likely that the legal and practical times would for the most part be allowed to drift, at least until this caused notable problems and quite likely for some time thereafter. Examples of the problems arising, taken from the 1847-1880 period when legal time in Great Britain was local time but for most purposes Greenwich time, as used by the railways, was used, may be found in Howse's book (Greenwich Time and the Longitude). In 1858 the case of Curtis v. March (where a court wrongly disposed of a case because the defendant had not appeared on time by the court clock, on Greenwich Time, but appeared on time by local time) lead to the case law that legal time was local time (though almost all clocks showed Greenwich time). Despite the confusion so caused, and the lack of ready availability of local time, the matter was not legislated upon until 1880. It seems plausible that similar disputes, about whether or not something was done within specified times, could arise if UTC and UT1 were substantially decoupled.

While it would be clearly desirable for legislation in all countries to establish the legal time as being based on the readily available time scale, UTC, rather than GMT, I do not expect this state to be approached for several decades if it becomes practically important, or much longer if it does not.

A. If the appropriate international bodies decide to eliminate the insertion of new leap seconds, would you foresee any practical problems for your institution/instrument/observations?

I would expect the problems described above to arise somewhere for some people, to cause confusion in areas where exact timing is legally important, and so to provide a source of undeserved extra income to lawyers. I would not expect to be involved in any such problems myself. It would be appropriate to study what legal definitions of time scales are used around the world, and whether eliminating leap seconds would cause any change in such definitions.

B. Would you be in favor of such a proposal?

My preference would be for Markus Kuhn's proposal of leap seconds announced several decades in advance. -- Joseph S. Myers jsm28@cam.ac.uk