The start of this new year started with some buzz about a leap
second being introduced between Dec 31st 2008, 23:59:59 and Jan
1st 2009, 00:00:00. I've had people ask where this leap second
actually comes from, and whether you need to worry about it in
your applications. To understand leap seconds means,
unfortunately, understanding how time is actually kept. There
are many different time keeping scales. They are either defined
as an arbitrary value, or obtained from astronomical
observations. First of all, there are the variants of Universal
Time. UT1 is the principal form of Universal Time. UT1 is the
same all over the world, and is related to the Earths rotation.
The rotation speed of the Earth is not precise, and is slowed
down by the tidal friction of the Moon among things. Obviously,
this is a bad base to build a stable and precise timekeeping
scale on. An (SI) second is nowadays defined as "the duration of
9 192 631 770 periods of the radiation corresponding to the
transition between the two hyperfine