The "C" key (and Shift-C) are currently documented as Clock (show for 10 sec) and (Hold).
However, that doesn't currently work if "Show Crystal" is disabled in the Program settings.
Most users aren't even aware of the keyboard shortcuts and just let the Aquarium run at default settings. Most users have never even seen the Settings panel.
I suppose a message could be displayed in white text when the user presses C and the Crystal is hidden "Please enable the Crystal feature if you wish to display the clock."
But then we would get 4-5 tech support e-mails a month asking how to avoid getting this message and 1-2 asking how to enable the Crystal feature.