3.. 2.. 1.. 1.. 0... Happy New Year!

On the 31st December, the day will be one second longer (seems an odd choice of day to me, why put the extra second on the one day when everyone is clock-watching?)



SERVICE DE LA ROTATION TERRESTRE OBSERVATOIRE DE PARIS 61, Av. de l'Observatoire 75014 PARIS (France) Tel. : 33 (0) 1 40 51 22 26 FAX : 33 (0) 1 40 51 22 91 e-mail : services.iers@obspm.fr http://hpiers.obspm.fr/eop-pc

Paris, 4 July 2008

Bulletin C 36

To authorities responsible for the measurement and distribution of time

UTC TIME STEP on the 1st of January 2009

A positive leap second will be introduced at the end of December 2008. The sequence of dates of the UTC second markers will be:

2008 December 31, 23h 59m 59s 2008 December 31, 23h 59m 60s 2009 January 1, 0h 0m 0s

The difference between UTC and the International Atomic Time TAI is:

from 2006 January 1, 0h UTC, to 2009 January 1 0h UTC : UTC-TAI = - 33s from 2009 January 1, 0h UTC, until further notice : UTC-TAI = - 34s

Leap seconds can be introduced in UTC at the end of the months of December or June, depending on the evolution of UT1-TAI. Bulletin C is mailed every six months, either to announce a time step in UTC or to confirm that there will be no time step at the next possible date.

Daniel GAMBIS Head Earth Orientation Center of IERS Observatoire de Paris, France


The last time this happened was at the end of 2005, and before that was 1998. The most up to date information is always given in Bulletin C. The history of modifications is listed here and here. The DUT bulletins give information about the difference between UCT and UT1 (i.e. time based on rotation of Earth alone).

The reason for this is that the earth's spin is irregular, affected by gravitational pulls of nearby objects. In addition, the tidal effect of the moon gradually lengthens the day. This means that without correction the earth would gradually go out of sync with the Earth's rotation (a more extreme version of this lead to the introduction of the leap year with the Gregorian Calendar).

The decision was made to keep the standard time within 0.9 seconds of the Earth's time, and this is monitored in Paris by the IERS, of l'observatoire de Paris. Having a standard time is very important, as an accurate timestamp is important in many modern applications, one such example being GPS. What that time standard should be is a matter of lesser importance.

The trouble with the current standard is the arbitrary shifts, which happen at irregular intervals (such as on the 31st December). This can make it hard to cope with in certain applications. Therefore in 2003 a meeting was held to discuss the future of timekeeping. The proposal is that the broadcast standard time signal would no longer have 'leap seconds'. As a result of this, the broadcast time would gradually slip out of sync with the Earth. This would not be noticeable for social reasons for a long time, generations, but eventually midnight on the standard time would slip to being in daylight. The correction would be dealt with by introducing a 'timezone'-like correction. I.e. clocks should be synchronized to standard time minus 3 seconds, then standard time minus four seconds. All machines would use standard time (as they use UCT now) for talking to themselves and to each other, but would add the appropriate correction (plus the needed time shift due to longitude) before talking to humans.

As an example, this website is running on a machine which thinks it is 7 hours earlier than it actually is. The machine is running on UCT (which as I write is 10:30am), but the machine reports the time is 03:30 when I use the unix date command. The WordPress software I'm using now has been told what my timezone is, and it makes a different correction to UCT, and reports the time correctly (for me). Allowing 'standard time' to drift would mean that the machine would make a correction of a few more seconds before reporting the time to the human. This would make it easier for machines to do date based calculations (as they wouldn't have to worry about the time shifts) and would allow the relevant corrections to be made once they were needed. The proposed date for this change is by 2022.

Of course, only folks whose timezone is currently the same as UCT will 'see' this leap-second as the year ticks over (i.e. London, Lisbon and places in the same timezone). Other locales will have their leap seconds at the corresponding local time (the whole world changes at once). I.e. 01:00 in Madrid and Paris, 05:30 in Delhi and 19:00 in New York. (I know that London and Lisbon aren't the same timezone, London uses GMT in winter, and Lisbon uses WET, but what is the difference between them?)

I hope you enjoy your 'extra' second!