Since the last update to Mac OS X Snow Leopard (10.6.3) the time has disappeared from my menu bar when I go into the Date & Time pane in System Preferences and click on the 'Show date and time in menu bar' it doesn't let me. Is this a noted bug? and can it be fixed easily?
Try Logging out, use a different user, and move the ~/Library/Preferences/com.apple.systemuiserver.plist out of your ailing user directory. Then Log back in. I suspect everything will be fine then.