In Czech Republic, this change will take place the following day, i.e. July 1, two hours after midnight - the Czech Republic is in the time zone UTC + 1, and it will also be influenced by the summer time (+1hr).
What is a leap second?
One second is inserted (or can theoretically be also removed) into the atomic scale UTC so that the scale would not astray from the astronomical scale UT1 which is tied to the Earth's rotation. IERS follows the deviation between UTC and UT1 if it is greater than 0.9 s, reports the appropriate change by the use of leap seconds.
When a positive leap second is inserted into UTC (added), then the clock will show the following time change:
23:59:59,
23:59:60,
00:00:00.
In case of a negative leap seconds clocks show:
23:59:57,
23:59:58,
00:00:00.
The first leap second was added in 1972. This year's leap second will be the 26th since then.
More information about leap seconds, atomic and astronomical time scales and their creation can be found in the Metrologie magasine on the link bellow (in Czech only).
http://www.unmz.cz/files/metrologie/casopis/priloha-metrologie-2013.pdf
