UI-Only field for time calculation

I want to create a UI-Only field that calculates NOW + 30 mins or 1 hour. Can I do that in the Formula section of the field creation?

You can create custom field renderer snippet. Does it also need to stay current? ie. stay 30 minutes ahead of now or 30 minutes ahead of when it was rendered?

Yes, would be great if it can always stay current!  Can you give an example of how this would be done?