It's Gnome based on Debian Stable. For the configuration I use the tool 'gnome-tweak-tool'. By the way, thanks for pointing out 'Timewarrior'. Here are my first steps with the tool. I like the 'summary' feature. It's exactly what I need. I'll use Timewarrior and Taskwarrior the next time to see if they complement each other.

user1 @ terra: ~ $ history | grep -i timew
868 sudo apt-get install timewarrior
870 man timewarrior
874 apt-cache show timewarrior
875 man timew
876 timew start
878 timew start java basics
879 timew bases summary
883 timew stop
884 timew summary
885 timew java summary
886 timew tags
887 timew help
888 timew show
889 timew help | less
890 timew week
993 timew
995 history | grep -i timew
user1 @ terra: ~ $