The Clock app on my Sony Xperia Z5 running Android 7.1.1 crashed every time I tried using the stopwatch or the countdown timer.
Neither rebooting the phone nor changing date formats helped. In the end I had to delete everything stored on behalf of the app. Next, I had to reenter my alarms on the alarm tab and a handful of assorted timezones on the world clock tab.
The only thing I can think of causing the crash is that I ran the countdown timer last week while Do not disturb was active. I’m not sure who to blame, Google, Sony, or myself.