16h 57m
A quick search for ios screen time wrong showed that a wildly wrong time is a common problem. Turning the phone off and back on didn’t help. Nor did deleting the widget, turning Screen Time off and back on, and re-adding the widget. After trying that fix, my phone showed a never-changing 30s in the widget and 14h in the app.
What finally worked: turning Content & Privacy Restrictions on and back off.