Dynamic Changing Overlay

The title is technically a lie, I just don't know how to properly describe it in 3-4 words. So, in my mod, there's gonna be a barometer that shows your coords, and the clock shows the in-game time. 

1. How do I make the overlay change with your coords and game time
2. How do I make it only show with the specific items
3. Are there any other semi-useless items I could give uses

Thank ya'll in advance!