Reading one of the posts about carrier related is interesting, but I think it's just coincidental. I switched from T-Mobile to Verizon right around the time I started experiencing the problem.
Here's where I am now. I believe it to be the Goole Keyboard Gboard or maybe just a 3rd party keyboard. As soon as I take Gboard off the problem doesn't return. I had the problem on the iPhone 7+. I upgraded to the iPhone X over the weekend and thought I'd give the Gboard a go and sure enough, the clock disappeared shortly afterwards. I removed it and so far it hasn't returned. Too bad, I really enjoy the Gboard keyboard.
I'm going to try another 3rd party keyboard.