I had a problem with my menu-bar clock and date after some wrong fiddling the clock disappeared. I couldn't figure out how to reinstall it. I did see receive some suggestions, but none of them worked and after I tried them it got worse: the menu-bar disappeared completely!
One of the suggestions is "You need to reset the per-user configuration of the notification area." by David Foerster. I would like to know how to do it. BTW, I am using Gnome Classic.