I like this app: ATracker PRO - Daily Task and Time Tracking by WonderApps AB https://appsto.re/us/Ab-9E.i

You set up some tags and tap on them to start the timer. For example I track things like meeting times or time spent in code review. Also generates reports and gives you JSON to write custom apps with.

By the way, I don't have any affiliation with the app or company, I just think it's a well designed app