How can I bring back my world clock widget?
Go to System Preferences → Extensions and click on ‘Today’ to see the World Clock widget.  Make sure it has a check next to it.  When you go back to Notification Center, you should see the clock.

I've had problems where some of my widgets would disappear for no obvious reason.  I don't know why this happens but I always end up going back to Extensions to add them back.