| Jurisdiction | date legally adopted UTC |
number of legally counted leap seconds |
legally elapsed seconds since 1972-01-01 |
| UTC and TAI | duUTC | nlUTC | etUTC |
| Germany | duGermany | nlGermany | etGermany |
| France | duFrance | nlFrance | etFrance |
| New Zealand | duNewZealand | nlNewZealand | etNewZealand |
| Australia | duAustralia | nlAustralia | etAustralia |
| Quebec | duQuebec | nlQuebec | etQuebec |
| United States of America | duUSA | nlUSA | etUSA |
| GMT, UT, POSIX Canada (not Quebec) |
N/A | 0 | etGMT |
In the table above time is from the javascript Date() of
the web browser. It is only as accurate as the system clock and
configuration.
Because javascript does not strictly conform to UTC this
demonstration will not be correct during a leap second.
This table displays the futility of producing a single algorithm by which an epoch-based time scale can agree with the time scales in contemporary use.
This demonstrates that the question
What time is it?is very different than the question
How many seconds have elapsed?In practice the answer to the first question has been the same, to within a second, everywhere on earth, for over a century.
To put it another way:
It is much easier to achieve worldwide agreement on what time it is right now than to achieve agreement on the meaning of every elapsed second throughout history.
Canada has an interesting situation starting 2009-01-01, for
Quebec has adopted UTC, whereas the standard time acts of other
provinces still refer to GMT.
The situation is even more confusing in some european countries.
Some versions of the legal documents refer to GMT in one official
language, and to UTC in another official language.
Equating UTC with GMT was understandable from the era when only the national metrology agencies had the sorts of chronometers which could distinguish between mean solar seconds and SI seconds, and keep track of a continuous count of SI seconds over long intervals. Prior to the 1960s all national metrology agencies had reset the national clocks ad hoc whenever their observatories indicated that to be necessary.
They differ because of the different dates at which each
jurisdiction legally adopted UTC as the basis for standard time.
Prior to that date the legal time of each jurisdiction was
counting mean solar seconds. There are no leap seconds in
mean solar time.
Subsequent to that date the legal time of each jurisdiction
began to count SI seconds based on TAI, with the leap
seconds added into UTC.
There is an important but rather pedantic distinction between the number of legally elapsed seconds and the number of officially elapsed seconds. In almost all jurisdictions the national metrology agencies, observatories, and/or radio broadcasts interpreted their organizational mandate as allowing the use of UTC instead of GMT. Thus for official purposes they adopted UTC as of 1972-01-01, but this did not modify the legal time in legislative acts, statutes, etc.
(Denmark is a notable exception to the above. Danish law asserts that legal time is based on the Greenwich meridian. The Danish national metrology agency interprets the law so strictly that they have declined to operate an NTP server, for that would be providing UTC instead of GMT.)
In 1884 the International Meridian Conference resolved that the "Universal Day" was a mean solar day, and in 1928 the IAU named "Universal Time" as a subdivision of the Universal Day.
The advent of cesium atomic chronometers made it possible to coordinate the synchronization of worldwide radio broadcasts for the first time ever. Leap seconds were invented for UTC in order to broadcast seconds of uniform length without abrogating resolution V of the IMC.
In 1975 the General Conference of Weights and Measures (CGPM) resolved that UTC provides mean solar time and as such endorsed its use for civil time. Without leap seconds UTC would not satisfy this resolution, and that would be abrogation of the Metre Convention.
Each jurisdiction has its own sovereign time scale. Various national systems for time distribution broadcast notably different times until the 1960s. Until 1988 it was the mandate of the Bureau International de l'Heure (BIH) to monitor those broadcasts and publish the differences. Since 1988 the International Bureau of Weights and Measures (BIPM) monitors the differences between the various national versions of UTC which serve as the official time of each jurisdiction. Every month the BIPM publishes the differences between the various versions of UTC in Circular T. In some countries the national version of UTC(k) consistently differs from most others by many microseconds.