Hi Vaclav,
The length of the day is very, very, very slowly becoming longer and one day (assuming no outside forces act) it will end up at 25 hrs -- and longer. There are several forces, some of which effectively act in opposite directions that change the Earth's rotation period over extremely long time scales.
There is a Wiki article here that explains the guts of what has happened and why here:
http://en.wikipedia.org/wiki/Tidal_acceleration
Hope that helps.
Best,
Les D