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

This means that tomorrow, 3:59:59 PM PST will be followed by 3:59:60 PM PST prior to the advent of 4:00:00 PM PST.

Previously, previously.