I realized a second strange thing after upgrading from 8.04 LTS to 10.04. My (GNOME) panel contains the clock applet. This applet shows current date and time. The time is not upgraded after I start a GNOME session. When I remove the applet and insert it again, it works fine. But at some point it stops again. Has anyone seen this? Is there a workaround? Several other people seem to have the same problem, but as far as I saw it they had no solution.
In a bug report on this issue one commenter says that a workaround for another bug fixed the clock freezing problem for him. The workaround delays the startup of gnome-panel for three seconds. (Strange, eh?)
Here is the workaround: