Summer Time Start and End Dates.

In the UK, as with all the other countries that are part of the European Union, we now always change the clocks at 01:00 UTC on the last Sundays in March and October.  Summer Time starts, and the clocks will therefore be going forward an hour (you lose an hour's sleep!), on the night of Saturday 26th and Sunday 27th March 2005.  Summer Time will end on the night of Saturday 29th and Sunday 30th October 2005.

In February 2001, the European Union agreed to stay with this last Sunday in March and October timing for the next five years from 2002 to 2006, and requires all member states to abide by this decision.  The British Parliament formerly adopted this in late May 2001 - see the Greenwich Mean Time Web Site.  The following table shows the start and end dates for the current decade.  The European Union Parliament has stated that it wants to specify these dates sufficiently far ahead to allow everyone to be able to plan ahead, but doesn't intend to specify the change dates for 2007 onwards until early in 2006.  The final years of the decade in the table below are therefore speculative, and this page will be updated as soon as Europe makes up it's mind!

Leap Seconds.

Many factors affect the rotational speed of the Earth, including tides, and the gravitational pull of the Sun and Moon, etc.  Since the creation of the Caesium atomic clock in the 1950's, it has been appreciated how relatively irregular the Earth's rotation actually is.  Atomic clocks are now used throughout the World, but they need correcting at irregular periods in order that they match the Earth's rotation.

In 1967 the second was defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.  However, since then the Earth has been turning, on average, a little slower.  Extra seconds - "leap-seconds" - are inserted into our time when it becomes too far ahead of the Earth.  This timescale is called UTC - "Co-ordinated Universal Time", and is kept within 0.9 of a second of the Earth's actual rotational time by the International Earth Rotation Service.  Leap-seconds are usually only added after the last second of the last day of June or December, but in exceptional circumstances could also be added in March or September.

The IERS have recently announced their predictions & they currently show that there will be no need for a leap second at the end of December 2005.

(The last leap-second was on December 31st 1998.)