Changing the date and time in KDE can result in this error sometime. Here is how to workaround it.
1: Go to the NTP pool project site. http://www.pool.ntp.org/en/
2a. There will be a list of continents on the side from which you can narrow down to the country in which you are situated.
2b. Take note of the URL
3. Open terminal and login as root.
4. Run (without quotes) "ntpdate (URL of country)"
5. For example. "ntpdate de.pool.ntp.org" for Germany.
6. Your clock should be set.
I hope that this helps you.