I have not tried chessclocks. However I have been experimenting around making a "timeplan" on a scenario in excell.
I have used it when playing with a friend of mine who is relatively new and a very slow player.
It actually worked very good by giving us an estimation on availiable time for attacker setup/defender setup and for each player turn.
Drop me a mail and I will send it over. My mail adress is firstname.lastname@example.org.