How can I add a UTC world clock to my OSX dashboard?
There is already a UTC clock available.

Assuming you're on Yosemite: 


Go to your notification center  
Click Edit at the bottom  
Add the World Clock and click Done.  
Hover over the World Clock you just added, and click on the (i) icon.  
Enter UTC when it asks for City.   
That's it. You're done.