Emerald Time (https://emeraldsequoia.com/et/index.html) was my favorite clock-setting app. Always fun to see the variation among sources. I was sad to see the company shut down.
Back on New Years Eve 2016 I wanted to see once in my life one of these leap second which got inserted every few years. Emerald Time was the only clock app I found which displayed the deciseconds: https://imgur.com/a/r1d6OkW
(00:59, because of UTC+1)