Well, the Clock and Calendar are both built into Android, so when you first get a new phone, there are already certain functions on it (things like phone, contacts, calculator, clock and calendar). So if you installed a different clock or calendar app from the Play Store, it's likely that you're trying to uninstall the stock versions... and you can't. Go to Play --> My Apps and scroll through for the apps you don't want. You should have the option to uninstall there.