How do I get my dual clock back on Samsung Galaxy S7 Edge on Android 6?
I believe that the Dual Clock only works when you are roaming. I could be wrong of course, but last time I left the UK the Dual Clock only appeared when I was connected to an internet connection and was in the location; when I got back into the UK the Dual Clock completely disappeared, and it hasn't turned on since. Hope this helps :)

If you're happy to not have Dual Clock you could just check the Clock app; but I'm sure you're not happy otherwise you wouldn't have posted this.