Hide clock on lock screen in Windows 8
Actually, there is a way to hide the clock on the lock screen. This guide shows you how to customize the look of the clock on the lock screen; you can hide the clock by changing the value data to a simple space, or to something else if you wish.
Windows-L will still lock the desktop and require the user to put in a password (if configured).
There is no way to hide the clock from the lock screen or "Charms" screen. You can (on the Personalization page in Settings) customize what appears below/around the clock on the Lock screen.